BritishAmerican System of Units
Discussion
introduction
In like manner there were natural measures of quantity, such as fathoms, cubits, inches, taken from the proportion of the human body, were once in use with every nation. But by a little observation, they found that one man's arm was longer or shorter than another's, and that one was not to be compared with the other, and therefore wise men who attended to these things would endeavour to fix upon some more accurate measure, that equal quantities might be of equal values. Their method became absolutely necessary when people came to deal in many commodities, and in great quantities of them.
This intro should talk about the cultural origin of these traditional units, but I haven't decided what to write yet. Let's just say they evolved in ways that were more organic and less logical than the SI units. The rest of the discussion follows this sequence: traditional units (length, mass, area, volume), nonmetric scientific units (the footpoundsecond systems), and then let's try to end it.
length
The English system is composed of a lot of sensible length units. Hands, feet, rods, paces — these are things most of us can relate to. Furlongs, fathoms, miles, yards — these make sense if you know a little bit of etymology (the study of the origin and evolution of words). Unfortunately, the conversion factors are a mess. Feet don't fit into furlongs in an easy to grasp way. There are a lot of "nice" numbers in this system — numbers like 3, 4, 5, 6, 8, 12 and 16 — but after a while the "nice" numbers put you through combinatoric gyrations that bring computational pain and suffering.
 thou
 One thousandth of an inch. Called a mil in the United States. The plural of thou is thou. One thousand thou equal one inch. The plural of mil is mils. One thousand mils equal one inch.
 inch
 An inch was originally the width of a man's thumb, but was later defined as the length of three barleycorns placed end to end. The word inch comes from the Latin word for onetwelfth (uncia). The Romans brought the concept of the 12 inch foot to England when they invaded in the year 43 and left it behind when they were expelled in the year 409. The inch is commonly subdivided into halves, quarters, eighths, sixteenths, and other powers of two; but can also be divided into hundredths (as in the caliber of firearms) or thousandths (called thou in the UK and mils in the US). One inch is now defined as exactly 25.4 mm.
 hand
 A hand is the width of a man's hand measured across the palm and including the thumb. It was traditionally used to measure the height of horses and not much else as far as I can tell. A standard hand is 4 inches.
 foot
 A foot is the length of a man's foot — a convenient measuring tool for men with feet. A standard foot is 12 inches or 304.8 mm exactly.
 cubit
 A cubit is the distance from the elbow to the tip of the middle finger on a man's arm. The name is derived from the Latin word for elbow (cubitum). The cubit is an ancient unit that has varied somewhat over time and place. The Roman cubit was 17.47 inches long, the Greek 18.20 inches, the Sumerian 20.42 inches, and the Egyptian 20.6 to 20.8 inches. The English cubit is 18 inches.
 yard
 A yard is the length from the King's nose to his outstretched hand. Presumably, after the king held out his hand someone placed a stick in the gap and marked it. This stick would then be the standard stick of the kingdom. Yard is an Old English word for staff, rod, or stick. That makes the word yardstick a candidate for the Department of Redundancy Department since a yardstick is literally a stickstick. A standard yard is 3 feet long. Since the International Yard and Pound Agreement of 1959, the yard has been defined as exactly 914.4 mm. This number was a compromise between the British and American definitions and also gives nice round values for the foot (304.8 mm) and inch (25.4 mm).
 pace
 The pace has its origins in Rome. The passus was measured from the heel of one foot to the heel of the same foot when it next touched the ground. This is a convenient unit for measuring walking distances (again, for men with feet). A standard pace is 5 feet long.
 fathom
 The fathom is a measure of length that was commonly used by navigators. It was the length to which a man could extend his arms while measuring ropes used to determine the depth of navigable waters. The word fathom has its roots in the Old English word for embracing arms (fæðm). A standard fathom is 6 feet long.
 rod
 A rod is a measure of length equal to 16½ feet or 5½ yards. It is also called a pole or a perch. (I'd hate to meet the budgie that needed a sixteen and a half foot perch.)
 chain
 Surveyors commonly used chains for measuring distances. The most famous of these was developed by the English mathematician Edmund Gunter (1581–1626). The links of Gunter's chain were each 7^{92}_{100} inches long. One hundred links gave it a total length of 792 inches, 66 feet, or 22 yards. Not a sensible number if you ask me, but then I don't play cricket. The distance between wickets on a cricket pitch is 22 yards.
 furlong
 Literally, the length of a furrow (the trench made in the ground by a plow). A sensible length for farmers that later evolved into the acre, which is discussed later in this section. A standard furrow is 220 yards long or ⅛ mile
 mile
 Like the inch, this word is a remnant of the Roman conquest of Britain (and since it appears in so many other languages, the Roman conquest of many other places). One mile was the distance of a thousand paces — in Latin, mille passus. A pace being 5 feet gives a mile of roughly 5000 feet. The mile acquired its current value of 5280 feet (1760 yards) by the decree of the English parliament during the reign of Elizabeth I. Since this was a legal definition it became known as the statute mile — statute being another word for law.
The distance called a mile varies greatly in different countries. Its length in yards is…
 Austria, 8,297 yards
 Brunswick, 11,816 yards
 England and US, 1,760 yards
 Hungary, 9,139 yards
 Italy, 2,025 yards
 Netherlands, 1,094 yards
 Norway, 12,182 yards
 Poland, 8,100 yards
 Prussia, 8,238 yards
 Spain, 1,552 yards
 Sweden, 11,660 yards
 Switzerland, 8,548 yards
You can't walk on the ocean so sailors developed a variation on the thousand paces concept. A nautical mile was originally defined as the distance spanned by one minute of arc measured on a meridian of the Earth — basically ^{1}_{60} of ^{1}_{360} of the Earth's circumference from one pole to another and back. A trip around the world in this fashion is then 21,600 nautical miles.
Since the Earth is a slightly flattened sphere (an oblate spheroid) a trip around the equator is 0.2% longer than a trip around the poles. (Using the equator as the standard gives a variation on the nautical mile called the geographic mile.) This small difference is important to ships, planes, and spacecraft traveling long distances. A 0.2% error over the width of the pacific ocean is about 20 miles (20 statute miles).
For simplicity's sake, the nautical mile is currently defined as 1852 m exactly, which works out to approximately…
1 nautical mile
1852 m 1 inch 1 foot 1 0.0254 m 12 inches 6076.11549…feet
This definition was proposed in 1929 at the incredibly named First International Extraordinary Hydrographic Conference (la première Conférence hydrographique internationale extraordinaire) and adopted by many governments shortly thereafter. Three exceptions to this were the UK, US, and USSR who all decided to wait awhile. It was incorporated into the International System of units as an acceptable nonSI unit subject to review until 2018 when it was quietly dropped. It is still used for marine and aerial navigation — and for some reason NASA likes it. There appears to be no standard symbol to represent this unit. The symbols M, NM, Nm, and nmi are all used for the nautical mile.  league
 A league is usually thought of as the distance a person could walk in an hour — 3 miles. On land, that would be 3 statute miles. At sea, 3 nautical miles is called a marine league. In other countries, well… again I refer to Webster's 1913 Dictionary.
A measure of length or distance, varying in different countries from about 2.4 to 4.6 English statute miles of 5,280 feet each, and used (as a land measure) chiefly on the continent of Europe, and in the Spanish parts of America. The marine league of England and the United States is equal to three marine, or geographical, miles of 6,080 feet each. Note: The English land league is equal to three English statute miles. The Spanish and French leagues vary in each country according to usage and the kind of measurement to which they are applied. The Dutch and German leagues contain about four geographical miles, or about 4.6 English statute miles.
The league was never adopted as a practical unit in England, which is evidenced by Webster's generally vague definition. It survived mainly as a poetical or rhetorical device as in Alfred, Lord Tennyson's narrative poem, The Charge of the Light Brigade.
Half a league, half a league,
Half a league onward,
All in the valley of Death
Rode the six hundred.
"Forward, the Light Brigade!"
"Charge for the guns!" he said:
Into the valley of Death
Rode the six hundred.The metric league (la lieue métrique) of exactly 4 km was used in France during the 19th century. It appears in the the title and body of Jules Verne's adventure novel, Twenty Thousand Leagues Under the Sea (Vingt mille lieues sous les mers), and refers to the distance the story's narrator traveled with Captain Nemo in the submarine Nautilus. Since the meter was originally defined so that the Earth's circumference would be 40,000 km, the Nautilus apparently circumnavigated the globe twice.
Me croiraton? Je ne sais. Peu importe, après tout. Ce que je puis affirmer maintenant, c'est mon droit de parler de ces mers sous lesquelles, en moins de dix mois j'ai franchi vingt mille lieues, de ce tour du monde sousmarin qui m'a révélé tant de merveilles…. Shall I be believed? I do not know. And it matters little, after all. What I now affirm is, that I have a right to speak of these seas, under which, in less than ten months, I have crossed 20,000 leagues in that submarine tour of the world, which has revealed so many wonders. Jules Verne, 1871 Jules Verne, 1871
year  nation  definition  

1893  American 

0.91440 

1959  International 
 0.9144 m  
British 

0.91399 
units  conversions  

1  thou  = 0.001in 
inch [in]  = 0.0254 m (exactly)  
1  hand  = 4 in 
1  foot [ft]  = 12 in 
1  cubit  = 18 in 
1  yard [yd]  = 3 ft = 36 in 
1  pace  = 5 ft = 60 in 
1  fathom  = 2 yd = 6 ft = 72 in 
1  rod  = 5½ yd = 16½ ft = 198 in 
1  chain  = 4 rods = 22 yd = 66 ft = 792 in 
1  furlong  = 10 chains = 220 yd = 660 ft = 7920 in 
1  statute mile [mi]  = 8 furlongs = 1760 yd = 5280 ft = 63,360 in 
nautical mile  = 1852 m (exactly) = 6076.12 ft (approximately)  
1  league  = 3 miles = 5280 yd = 15,840 ft = 190,080 in 
Let's move on to…
mass (or is it weight?)
Actually, it's both. The English units for mass are also the units of weight for that mass in a standard gravitational field (9.806 65 m/s^{2} exactly). This part of the English system should probably be called the French system since so many of the units have their origins in France. There were two paths: avoirdupois and troy.
 Most commodities use the avoirdupois system of mass units. The term was adapted from the French phrase "aveir de pois" or "aver de peis". Roughly translated means "goods of weight" or "goods [sold] by weight" to distinguish it from goods sold by the piece. The basis of the avoirdupois system is a pound with 16 ounces. One pound avoirdupois is now defined as 0.453 592 37 kg (or 453.59237 g if you prefer).
 Precious metals, gems, and drugs use the troy system; named, not for the ancient Greek city of Troy, but for Troyes, France where they were introduced. The troy ounce is supposed to have been brought from Cairo during the crusades and introduced at a fair in Troyes. They are also called apothecaries' weights from the Old French word apotecaire — a shopkeeper, especially one who dispensed medicines (a chemist in the UK or a pharmacist in the US). The basis of the troy system is a pound with 12 ounces.
These units do not seem to come in the same kind of easily relatable sizes like those for length. There are a lot of units that just seem to mean "small". This list is also shorter than the previous one.
 grain
 A grain was said equal to the mass of an average grain of wheat taken from the middle of an ear. There 7000 grains in the the avoirdupois pound and 5760 grains in the troy the pound.
 scruple
 From the Latin word scrupus, a small rough pebble or a chip of stone — basically, something small. A scruple is 20 grains.
 pennyweight
 At one time English pennies weighed 24 grains.
 dram
 A fraction of an ounce — an eighth or a sixteenth depending on the system. The word is derived from the ancient Greek coin the drachma (δραχμή). One drachma weighed about one dram. Dram also refers informally to a serving of whiskey, especially Scotch. In this context, one might translate dram as "a wee bit". An actual dram of whiskey would not be considered an adequate serving, however. When someone says this, they are being witty.
 ounce
 The word ounce has the same origin as the word inch — the Old English word for one twelfth: uncia. An inch is onetwelfth of a foot and an ounce is one twelfth of a pound. Well, sometimes. It could also be one sixteenth, but that's not the origin of the word. I think what's really going on is that in the olden days the world for twelfth was used indiscriminately for all sorts of small fractions with little regard for mathematical consistency. I think they had other things to worry about.
 pound
 Pound comes from Latin pondus for weight. The abbreviation lb for pound comes from the Roman unit libra (about threequarters of an English pound), which comes from the Latin libro, to weigh. A variant symbol had a bar drawn through the ascenders like this ℔. In cursive form, the symbol was reduced to two vertical and two horizontal strokes like this #. This symbol for pound lives on today as the pound key on the telephone (also known as the number sign, hash, or octothorpe).
 stone
 A unit usually used for bulk agricultural commodities and legally defined as equal to 14 pounds. In practice, however, the weight of a stone varied with the article weighed.
 glass: 5 lbs
 meat, fish: 8 lbs
 sugar, spices: 8 lbs
 wax: 12 lbs
 lead: 12 lbs
 iron: 14 lbs
 cheese: 16 lbs
 hemp: 32 lbs
 hundredweight
 Logically, a hundredweight should be a hundred of something — a hundred pounds would be my educated guess. This was the choice made in England way, way back and adopted by the United States at its founding. But what if you prefer the stone over the pound as your basic unit of weight? This was the case in England soon after the Americans left the Empire. The nearest multiple of a stone greater than a hundredweight is 8 stone or 112 pounds. This became the new hundredweight in England. To distinguish between the two, the original 100 pound hundredweight is called a short hundredweight or a cental while the newer 112 pound hundredweight is called a long hundredweight.
 ton
 The origin of this word is the Old English tunne — a big container. Brewers are the last people who still use this word (mash tun, lauter tun, fermentation tun). Later the word also came to mean the capacity of such a container and was used as a unit of both volume and weight. The volume unit was not as popular as the weight unit except in the railroad and shipping industries. Eventually it was decided that a ton would be a good name for two thousand pounds, since that's about the weight of water that a tun could hold. When the hundredweight changed in England, so too did the ton. America kept the unit at 2000 pounds while the English changed the unit to 2240 pounds. (2240 pounds is 160 stone, by the way.) As with the hundredweight, the American ton is called a short ton while the English ton is called a long ton. The similarly sized SI unit of 1000 kg is called a tonne in England or a metric ton in the United States. To intentionally misquote George Bernard Shaw, "England and America are two countries divided by a common unit system."
units  conversions  

1  grain [gr]  = 1/7000 pound avoirdupois 
1  dram [dr]  = 1/256 pound avoirdupois 
1  ounce [oz]  = 16 drams 
1  pound [lb, ℔, #]  = 16 ounces = 7000 grains 
pound avoirdupois [lb, ℔, #]  = 0.453 592 37 kg (exactly)  
1  stone [st]  = 14 pounds 
1  short hundredweight [cwt]  = 100 pounds 
1  long hundredweight [cwt]  = 112 pounds 
1  short ton [tn]  = 2000 pounds 
1  long ton [tn]  = 2240 pounds 
units  conversions  

1  grain [G]  = 1/5760 pound troy 
1  scruple [℈]  = 20 grain 
1  pennyweight [dwt]  = 24 grains 
1  dram [ʒ]  = 3 scruples = 60 grains 
1  ounce [℥]  = 8 drams = 480 grains 
1  pound [lb, ℔, #]  = 12 ounces = 5760 grains 
pound troy  = 5760/7000 pound avoirdupois 
area
Generating area units should be a straightfoward affair. Take the length units and square them. This gives us units like square inches, square feet, square yards, square miles and so on. We complete the list with two units from agriculture (the acre and the rood) and three quantities that relate to land surveying in the United States. These aren't really units, but I find them interesting nonetheless.
 acre
 The word acre is related to the word agriculture. Traditionally, an acre was thought to be as much land as a pair of oxen could plow in one day. When you plow, you make a trench in the soil called a furrow. One pass across a field would leave a trench one "furrow long" — a furlong. How many furlongs could you cut in a days work? Well I have no idea, but the value that was adopted as the standard in England was the 22 yard surveyors chain developed by Edmund Gunter (1581–1626) in the 17th century. An English acre is then one furrow long by one chain wide.
1 acre 1 furlong × 1 chain 220 yards × 22 yards 4840 square yards  rood
 The word rood comes from the word rod. A rood is like an acre, only one forth as wide — as wide as a rod instead of a chain.
1 rood 1 furlong × 1 rod 220 yards × 5½ yards 1210 square yards ¼ acre  subdivision
 Each square mile section of the Public Land Survey System was divided into four divisions or quartersections (½ mi × ½ mi = 160 acres) and then divided again into sixteen subdivisions or quarterquartersections (¼ mi × ¼ mi = 40 acres). This last unit became a popular size for tracts of farmland and lead to the Americanisms "back forty" (the parcel of land farthest from a farmhouse) and "forty acres and a mule" (compensation that was promised to slaves after the Civil War but never delivered — a metaphor for the failure of the Reconstruction).
 section
 A section is a survey tract in the United States of one square mile. It is the basic unit of the Public Land Survey System, which traces its origins back to the first decade after independence. You may recall that George Washington was a surveyor before he was a general in the Continental Army or the president of the United States. This system set up the rectangular coordinates that were used to identify property boundaries in the territories outside of the original Thirteen Colonies.
 township
 In the Public Land Survey System, groups of 36 sections in a contiguous 6 mile by 6 mile square are called townships. Many counties in the United States are a whole number of townships in size making their area easy to determine.
units  conversions  

1  square thou  = 0.000001 sq in 
square inch [sq in]  = 0.00064516 m^{2} (exactly)  
1  square foot [sq ft]  = 144 sq in 
1  square yard [sq yd]  = 9 sq ft = 1296 sq in 
1  square rod  = 30.25 sq yd = 272.25 sq ft = 39,204 sq in 
1  square chain  = 16 sq rods = 484 sq yd 
1  rood  = 2.5 sq chains = 40 sq rods = 1210 sq yd 
1  acre  = 10 sq chains = 160 sq rods = 4840 sq yd 
1  square mile [sq mi]  = 640 acres 
units  conversions  

1  subdivision  = 40 acres 
1  section  = 1 square mile = 16 subdivisions = 640 acres 
1  township  = 36 sections = 576 subdivisions = 23,040 acres 
volume
The unit systems preferred by scientists have volume units that are derived from length units. Take the meter [m] and cube it [m^{3}]. There you go. That's a unit. Don't use the cubic meter much? Alright. Try this. Take a tenth of a meter [0.1 m or 10 cm] and cube it [0.001 m^{3} or 1000 cm^{3}]. That's a unit. Call it a litre (or a liter). That's how the International System of Units and its predecessor, the metric system, work.
The English system does not work this way. Some might say it doesn't work at all. Most of the volume units in the English system were derived sensibly from available measuring devices — droppers, spoons, cups, pitchers, buckets, … containers of all sorts. Very few were derived mathematically by cubing length units.
English and American volume units diverged in 1824 over the definition of the gallon. The English wanted a gallon of water to weigh 10 pounds and parliament passed a law to make it so — the British Weights and Measures Act. The Americans stuck with the old standard that made a gallon of water nearly equal to 8 pounds by accident. This makes most English volume units about 20% larger than American units. Americans are often surprised when they see how satisfyingly large English pints of beer are. The units set before 1824 are rightly called English units, those set after 1824 are called imperial units, the English units that persisted in the United States are called US customary units.
Some volume units share names with mass units. This happened because the liquid dispensed was usually water based (milk, beer, cider, wine, vinegar, spirits, medicines). The measurer could be using the measuring device to measure volume or mass. The correspondence isn't all that strong, however. While one ounce of water has a mass that is nearly one ounce, one dram of water has a mass that is just kinda sorta close to one dram.
1 imperial fluid ounce of water  =  1.0022 avoirdupois ounces  ≈  0.2% error 
1 imperial fluid dram of water  =  0.9508 troy drams  ≈  5% error 
The correspondence falls apart with the ton. A tun (a type of container) holding a ton (a unit of volume) of water is much heavier than a ton (a unit of mass).
1 register ton of water  =  2.7869 long tons 
1 register ton of water  =  3.1214 short tons 
The mass of one imperial fluid ounce of water is nearly one avoirdupois ounce by design. When the imperial gallon was set at 10 avoirdupois pounds it was also divided up into 160 imperial fluid ounces. (The US gallon is divided up into 128 ounces.) Since 16 avoirdupois ounces make an avoirdupois pound, 1 imperial fluid ounce of water must have a mass of 1 avoirdupois ounce.
Well… not quite. The world pretty much runs on the International System. Both the UK imperial and US customary gallons are now defined in terms of the liter.
1 imperial gallon  =  4.54609 liters (by definition) 
1 US gallon  =  3.7854117843 liters (by definition) 
imperial/US  =  1.20094993… ≈ 20% difference 
This is the origin of the small difference in the UK and US fluid ounces.
1 imperial fluid ounce  =  28.4130625 milliliters (exactly) 
1 US fluid ounce  =  29.5735296… milliliters (approximately) 
imperial/US  =  0.96075994… ≈ 4% difference 
Grafting imperial units onto the International System caused them to drift a little further away from their intended values. To compare things, we'll use the traditional density of water — 1 g/cm^{3} or 1 g/ml.
1 g  28.4130625 mL  16 avoirdupois ounce  
1 mL  1 imperial fluid ounce  453.59237 g 
= 1.00224129…  avoirdupois ounce 
imperial fluid ounce 
Some volume units in the two systems are used only for measuring liquids (water, wine, beer, gasoline, oil, honey, extracts, tinctures), some only for loose dry commodities (flours, grains, fruits, nuts, tea, sugar, soil, gravel). Let's start with the liquid measures.
 minim
 The minim, as the name implies, is the smallest liquid measure in the English system — ^{1}_{60} of a dram by definition, about as much water as will form a drop.
 dram
 A dram is about as much liquid as one would take in a dose of medicine (the customary use of the word) or poison (the dramatic use of the word) or spirituous liquor (the humorous use of the word, since a dram is much too small to be considered a drink). The fluid dram is defined as ⅛ of a fluid ounce. A fluid dram of water weighs about as much as the dram of the apothecaries weight (to within 5% in imperial units), thus the conflation of the two measures in one name. Recall, that the apothecaries weight called a dram was derived from the name of a Greek coin, the drachma (δραχμή).
 teaspoon
 The approximate volume of a small spoon used for stirring and sipping tea. Would it have been called the "coffeespoon" if it was invented in the US? 1 imperial teaspoon is 1⅔ imperial fluid drams. 1 US teaspoon is 1⅓ US fluid drams.
 tablespoon
 The approximate volume of a larger spoon that would commonly be used at the table. It could also have been called a soup spoon, but it wasn't. A standard tablespoon is equal to three teaspoons.
 ounce
 A fluid ounce of water has a weight of about one ounce (to within 0.2% in imperial units). The exact size of a fluid ounce depends on whether you use the older US gallon (128 ounces) or the newer imperial gallon (160 ounces). More on this later. An imperial fluid ounce equals 4⅘ imperial teaspoons. A US fluid ounce equals 6 US teaspoons.
 gill
 A gill is ¼ pint, which is why it's also known as a quartern. That's 4 US ounces or 5 imperial ounces. The word is a corruption of gille, a kind of cup used to measure wine in France. Its ultimate origin was probably the Latin word for a small pot — gillo. The Latin word for a big pot became the English word gallon. Although it's spelled the same as the breathing organ of a fish, it's pronounced like the woman's name Jill.
 cup
 The volume of a typical cup for drinking water. A standard cup is 8 ounces — 8 customary ounces in the US and 8 imperial ounces in the Commonwealth of Nations.
 pint
 The word pint came to English from the Spanish word for a mark — pinta — probably a mark that was made on a larger measure. Exactly what that larger measure was is unknown to me, but my guess is something like a gallon. The connotation of the pint as a "small" unit carries over into the word pintsized. In both the US and imperial systems, a pint is ⅛ of a gallon but, as I stated before, the gallons aren't the same. 1 imperial pint contains 20 imperial ounces. 1 US pint contains 16 US ounces.
 fifth
 A fifth is the quantity of liquor equal to ⅕ of a US gallon, ⅘ of a US quart, 25⅗ US fluid ounces, or 757 milliliters (approximately). This is strictly an American unit. The origins of the fifth are lost to time. Some say it was a way to avoid onerous regulations that applied to sales of liquor in quart or larger amounts (the fifth was also known as a "short quart") but I can't find any references pointing to specific laws. Some say bottles with approximately this volume were the largest glassblowers could manage in one breath, but that's heresay not a quote. The fifth disappeared from US liquor stores in 1980 when federal regulations restricted the sale of distilled spirits to eight allowed metric measures. The closest in size to the fifth was 750 mL, which became an international standard of sorts.
 quart
 A quart is onequarter of a gallon. A somewhat obvious name.
 gallon
 The word gallon comes from the Latin galleta and refers to a standard container about the same size as a helmet. (The Latin word for helmet is galea.) A gallon is 4 quarts or 8 pints. A US gallon of 128 US ounces of water weighs about 8 pounds by accident. An imperial gallon of 160 imperial ounces of water weighs very nearly 10 pounds by design (8.34540449 pounds vs. 10.022417 pounds assuming the density of water is 1 g/cm^{3}). That's the whole source of contention. The English wanted a gallon that weighed something meaningful to them. Both gallons are now defined in terms of SI units: the US gallon is 3.785411784 liters and the imperial gallon is 4.54609 liters, exactly.
 barrel
 A unit barrel is about the equal to the volume of a typical wooden barrel. The size of a barrel is determined by what it contains (and also the year, but I won't get into that): 31½ US gallons of anything but petroleum, 36 imperial gallons of beer, 26¼ imperial gallons of wine, or 42 US gallons of petroleum.
 hogshead
 A hogshead is a container about twice the size of a barrel: 63 US gallons of anything, 52½ imperial gallons of wine, 54 imperial gallons of beer. The word is derived from the word for oxhead in Danish (oksehoved), Dutch (okshoofd), German (oxhoft), or Swedish (oxhuvud). What this has to do with anything is a mystery.
Now for the dry measures. Watch as we break the US volume units.
 peck
 2 gallons, 8 quarts, or 16 pints of dry material are called a peck. The origin of this word is uncertain. It could just be related to the word pick.
Peter Piper picked a peck of pickled peppers.
Did Peter Piper pick a peck of pickled peppers?
If Peter Piper picked a peck of pickled peppers,
Where's the peck of pickled peppers Peter Piper picked?  bushel
 4 pecks, 8 gallons, 32 quarts, or 64 pints make a bushel. The word is French in origin and refers to a container about a bushel in size. (Not a very informative word origin story.) The original English standard, called the Winchester bushel, was 18½ inches in internal diameter and 8 inches deep for a total of 2150.42 cubic inches approximately. The volume of a cylinder is an irrational number since π appears in its definition, which makes the Winchester bushel a difficult unit.
1 Winchester bushel = πr^{2}h 1 Winchester bushel = π(9¼ inch)^{2}(8 inch) 1 Winchester bushel = 2150.42017… cubic inch 1 Winchester bushel = 35.2390730… liter 1 imperial bushel = 8 imperial gallon 1 imperial bushel = 36.36872 liter (exactly) 1 US bushel = 2150.42 cubic inch (exactly) 1 US bushel = 35.2390702… liter 1 US bushel = 35.2390702… liter 8 US dry gallon = 9.30917749… US liquid gallon 1 US dry gallon = 1.16364719… US liquid gallon 1 US dry quart = 1.16364719… US liquid quart 1 US dry pint = 1.16364719… US liquid pint  quarter
 8 bushels make a quarter. This unit is only used for grain as far as I can determine. It's called a quarter probably because it's a quarter of a ton. Start from the true (but also false) assumption that "a pint's a pound the world around". The true part is that a US liquid pint of water weighs about a pound. That false part is that the US is the world. The questionable part is the assumption that dry commodities have the same density as water, which is wet.
1 pound 8 pint 8 gallon 8 bushel 1 US liquid pint 1 gallon 1 bushel 1 quarter = 512 pound = 0.256 short ton 1 quarter 1 quarter 1.2 pound 8 pint 8 gallon 8 bushel 1 imperial pint 1 gallon 1 bushel 1 quarter = 614.4 pound ≈ 0.274 long ton 1 quarter 1 quarter
A few US/imperial volume units are derived by multiplying length by width by height (or area times height).
 register ton
 The word ton comes from the archaic word tun, which was a kind of large container. Brewers are the only people who still use this word regularly. The register ton is a unit of volume (not mass) used in the railroad and shipping industries now equal to 100 cubic feet by definition. For example, a 50 foot long, 9½ foot wide, 13 foot tall boxcar has a capacity of 61¾ register tons and a 40 foot long, 8 foot wide, 8½ foot tall intermodal shipping container has a capacity of 27⅕ register tons. At one time, water with a volume of one ton had a mass of one ton, but the two units are now unrelated. A register ton of water weighs something like 3 tons (2.7869 long tons or 3.1214 short tons).
 cord
 1 cord of wood is eight feet wide, four feet high, and four feet deep, or 128 cubic feet; measured with a cord, thus the name.
 acre foot
 A good unit for reservoirs and other large scale water resources. Multiply the surface area in acres by the average depth in feet. Simplicity at its best. An acre is a furlong (660 feet) by a chain (66 feet). Multiplied by one more foot gives 43,560 cubic feet.
units  conversions  

cubic inch [cu in]  = 0.00001470612 m^{3} (exactly)  
1  cubic foot [cu ft]  = 1728 cu in 
1  cubic yard [cu yd]  = 27 cu ft = 46,656 cu in 
1  register ton  = 100 cu ft 
1  cord  = 128 cu ft 
1  acre foot  = 1613⅓ cu yd = 43,560 cu ft 
1  cubic mile [cu mi]  = 5,451,776,000 cu yd = 147,197,952,000 cu ft 
units  conversions  

1  minim [min]  
1  dram [dr]  = 60 min 
1  teaspoon [tbsp]  = 100 min 
1  tablespoon [tbs]  = 3 tsp = 300 in 
1  ounce [oz]  = 1⅗ tbsp = 4⅘ tsp = 8 dr = 480 min 
1  gill [gi]  = 5 oz 
1  cup [c]  = 8 oz 
1  pint [pt]  = 2½ c = 20 oz 
1  quart [qt]  = 2 pt = 5 c = 40 oz 
1  gallon [gal]  = 4 qt = 8 pt = 20 c = 160 oz 
gallon [gal]  = 4.54609 liters (exactly)  
1  peck [pk]  = 2 gal = 8 qt = 16 pt 
1  bushel [bu]  = 4 pk = 8 gal = 32 qt = 64 pt 
1  quarter [qr]  = 8 bu = 64 gal = 256 qt = 512 pt 
1  barrel [bbl]  = 26¼ gal (wine) = 36 gal (beer) 
1  hogshead  = 52½ gal (wine) = 54 gal (beer) 
units  conversions  

1  minim [min]  
1  dram [dr]  = 60 min 
1  teaspoon [tbsp]  = 1⅓ dram = 80 min 
1  tablespoon [tbs]  = 3 tsp = 4 dr = 240 min 
1  ounce [oz]  = 2 tbsp = 6 tsp = 8 dr = 480 min 
1  gill [gi]  = 4 oz 
1  cup [c]  = 8 oz 
1  pint [pt]  = 2 c = 16 oz 
1  fifth  = ⅘ qt = 25⅗ oz 
1  quart [qt]  = 2 pt = 4 c = 32 oz 
1  gallon [gal]  = 4 qt = 8 pt = 16 c = 128 oz 
gallon  = 231 cu in (exactly) = 3.7854117843 liters (exactly)  
1  barrel [bbl]  = 31½ gal 
1  barrel of oil [bbl]  = 42 gal 
1  hogshead  = 63 gal = 2 bbl 
units  conversions  

1  pint [pt]  
1  quart [qt]  = 2 pt 
1  gallon [gal]  = 4 qt = 8 pt 
1  peck [pk]  = 2 gal = 8 qt = 16 pt 
1  bushel [bu]  = 4 pk = 8 gal = 32 qt = 64 pt 
bushel  = 2150.42 cu in (exactly) = 35.2390717… liters (approximately) 
footpoundsecond
The footpoundsecond system is an attempt to make useful scientific units out of the mess that traditional English units evolved into. The foot is pretty good (since most people have two feet available for service). The second is very good (since it's an internationally recognized unit). But the pound, for lack of a better word, is bad. What is a pound? Is it a unit of mass or is it a unit of weight (and thus a unit of force)? To be precise, one should always indicate.
Start from the pound avoirdupois, the usual unit of mass and weight in the English system. SI dominates the world now and the English pound mass is now defined in terms of the kilogram.
pound mass = 0.453 592 37 kg
This value is exact by definition. It is not measured or calculated.
From here we move on to the first unit of force in the English system. Yes, you heard me right, the first. There are two — one in which the pound is a unit of weight and one in which it's a unit of mass. The pound force is defined as the weight of a pound mass in a standard gravitational field. Thus…
W =  mg 
pound force =  (pound mass)(standard gravity) 
pound force =  (0.453 592 37 kg)(9.806 65 m/s^{2}) 
pound force =  4.448 221 615… N 
The corresponding unit of mass is the horribly named slug. The slug is the unit of mass when the pound is a unit of force. A mass of one slug will accelerate at one foot per second squared when pushed by a one pound force.
F =  ma 
1 pound force =  (1 slug)(1 ft/s^{2}) 
(1 pound mass)(standard gravity) =  (1 slug)(1 ft/s^{2}) 
(1 pound mass) 
(1 slug)(1 ft/s^{2}) 
Thus…
slug = 32.174 048 6… pound mass
In SI units, this is approximately…
slug =  (32.174 048 6… lb)(0.453 592 37 kg/lb) 
slug =  14.593 902 9… kg 
And now for the second unit of force in the English system. The poundal is the unit of force when the pound is the unit of mass. A one pound mass will accelerate at one foot per second squared when pushed by a one poundal force.
F =  ma  
poundal =  (1 pound mass)(1 ft/s^{2})  
poundal = 


poundal = 


poundal =  0.031 080 95… pound force 
In SI units, this is exactly…
poundal =  (1 pound mass)(1 ft/s^{2}) 
poundal =  (0.453 592 37 kg)(0.3048 m/s^{2}) 
poundal =  0.138 254 954 376 N 
Now that we've sort of resolved the whole massweight debacle let's get on with this subsystem of the English system of units.
quantity  full name  symbol  

pound force  pound mass  
distance  foot  ft  
time  second  s  
speed  ft/s  
acceleration  ft/s^{2}  
standard gravity  32.174 048 6… ft/s^{2}  
force  pound force  lb (also lbf)  
poundal  pdl (lb ft/s^{2})  
mass  slug  slug (lb s^{2}/ft)  
pound mass  lb (also lbm)  
energy  ft lb  ft pdl  
power  ft lb/s  ft pdl/s  
moment of inertia  slug ft^{2} (lb ft s^{2})  lb ft^{2}  
torque  ft lb  ft pdl  
area  ft^{2}  
volume  ft^{3}  
mass density  slug/ft^{3}  lb/ft^{3}  
weight density  lb/ft^{3}  pdl/ft^{3}  
volume flow rate  ft^{3}/s  
mass flow rate  slug/s  lb/s  
weight flow rate  lb/s  pdl/s  
pressure  lb/ft^{2}  pdl/ft^{2}  
dynamic viscosity  lb s/ft^{2} (slug/ft s)  pdl s/ft^{2} (lb/ft s)  
kinematic viscosity  ft^{2}/s 
some more units
Blah, blah, blah. So many, many units. Base notes from the public domain Webster's Revised Unabridged Dictionary of 1913.
 horsepower
 A unit of power, used in stating the power required to drive machinery, and in estimating the capabilities of animals or steam engines and other prime movers for doing work. It is the power required for the performance of work at the rate of 33,000 English units of work per minute; hence, it is the power that must be exerted in lifting 33,000 pounds at the rate of one foot per minute, or 550 pounds at the rate of one foot per second, or 55 pounds at the rate of ten feet per second, etc.
 Btu
 {Mechanical equivalent of heat} (Physics), originally defined as the number of units of work which the unit of heat can perform, equivalent to the mechanical energy which must be expended to raise the temperature of a pound of water one degree Fahrenheit; later this value was defined as one {British thermal unit} (Btu). Its value was found by Joule to be 772 foot pounds; later measurements give the value as 777.65 footpounds, equivalent to 107.5 kgmeters. This value was originally called Joule's equivalent, but the modern joule is defined differently, being 10^{7} ergs. The Btu is now given as 1054.35 absolute joules, and therefore 1 calorie (the amount of heat needed to raise one gram of water one degree centigrade) is equivalent to 4.186 Joules.
 therm
 100,000 Btu
 quad
 quadrillion Btu
 candle
 {Standard candle} (Photom.), a special form of candle employed as a standard in photometric measurements; usually, a candle of spermaceti so constructed as to burn at the rate of 120 grains, or 7.8 grams, per hour. {Candle power} (Photom.), illuminating power, as of a lamp, or gas flame, reckoned in terms of the light of a standard candle.
 foot candle
 The amount of illumination produced by a standard candle at a distance of one foot.
 inches of mercury
 A unit of pressure
 Fahrenheit
 {Fahrenheit thermometer} is so graduated that the freezing point of water is at 32 degrees above the zero of its scale, and the boiling point at one atmosphere of pressure is 212 degrees. It is commonly used in the United States and in England.
212 °F = 100 °C and 32 °F = 0 °C
°F = ^{9}_{5}°C +32
°C = ^{5}_{9}(°F − 32)
 rankine
 an absolutetemperature scale related to the Fahrenheit scale
 knot
 (a) A division of the log line, serving to measure the rate of the vessel's motion. Each knot on the line bears the same proportion to a mile that thirty seconds do to an hour. The number of knots which run off from the reel in half a minute, therefore, shows the number of miles the vessel sails in an hour. Hence: (b) A nautical mile, or 6080.27 feet; as, when a ship goes nautical eight miles an hour, her speed is said to be eight knots. [1913 Webster] a unit of length used in navigation; equivalent to the distance spanned by one minute of arc in latitude; 1852 meters
 reyn
 Dynamic viscosity. 144 lb s/ft^{2}
Anything else? Don't answer that question.