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# International System of Units

## Summary

• The International System of Units is currently the generally accepted system of units in the physical sciences.
• The international abbreviation of the name is SI (from the French Le Système International d'Unités).
• The SI has four major components.
1. Seven defining constants (or reference constants) with exact values. These constants may be universal constants that arise from fundamental physical laws (Planck's constant h, Boltzmann's constant k, speed of light c), be connected to natural phenomena (hyperfine transition of a cesium atom ΔνCs, charge on a proton e), or have evolved from previous definitions of the base units (Avogadro's constant NA, luminous efficacy of green light Kcd).
2. Seven well-defined, dimensionally independent, base units that are assumed irreducible by convention (second, meter, kilogram, ampère, kelvin, mole, and candela).
3. A large number of derived units formed by combining base units according to the algebraic relations of the corresponding quantities (some of which are assigned special names and symbols and which themselves can be further combined to form even more derived units).
• The derived units are coherent in the sense that they are all mutually related only by the rules of multiplication and division with no numerical factor other than 1 needed.
• The derived units are also complete in the sense that one and only one unit exists for every defined physical quantity. Although it is possible to express many units in more than one way, they are all equivalent. (The converse statement is not necessarily true, however. Some units are used for more than one physical quantity.)
4. Twenty-four currently agreed upon prefixes that can be attached to any of the base units or derived units with special names creating multiples and division as needed. (The exception to this rule is the kilogram, which is already itself a multiple of the gram. In this case, prefixes should be added to the word gram.)
• The first three named multiples are the first three powers of ten (101, 102, 103).
Subsequent named multiples are larger than the previous named multiple by three orders of magnitude (106, 109, 1012, … ).
• The first three named divisions are the first three negative powers of ten (10−1, 10−2, 10−3).
Subsequent named divisions are smaller than the previous named division by three orders of magnitude (10−6, 10−9, 10−12, … ).
• Other scientific, traditional, and practical units and unit systems are still in use and are still useful.
The seven defining constants of the International System
description symbol value
hyperfine transition of a cesium atom ΔνCs 9 192 631 770 Hz
speed of light c 299 792 458 m/s
Planck's constant h 6.626 070 15 × 10−34 J s
charge on a proton e 1.602 176 634 × 10−19 C
Boltzmann's constant k 1.380 649 × 10−23 J/K
Avogadro's constant NA 6.022 140 76 × 1023 1/mol
luminous efficacy of green light Kcd 683 lm/W
The seven base units of the International System
quantity unit symbol
length meter m
mass kilogram kg
time second s
electric current ampère A
temp­erature kelvin K
amount of substance mole  mol
luminous intensity candela cd
Divisions of the International System
factor prefix symbol
10−1 deci d
10−2 centi c
10−3 milli m
10−6 micro µ
10−9 nano n
10−12 pico p
10−15 femto f
10−18 atto a
10−21 zepto z
10−24 yocto y
10−27 ronto r
10−30 quecto q
Multiples of the International System
factor prefix symbol
101 deca da
102 hecto h
103 kilo k
106 mega M
109 giga G
1012 tera T
1015 peta P
1018 exa E
1021 zetta Z
1024 yotta Y
1027 ronna R
1030 quetta Q