SI Unit Definitions
20 SI prefixes used to form decimal multiples and submultiples of SI units are defined in the tables below:
SI prefixes
| Factor | Name | Symbol |
|---|
| 1024 | yotta | Y |
| 1021 | zetta | Z |
| 1018 | exa | E |
| 1015 | peta | P |
| 1012 | tera | T |
| 109 | giga | G |
| 106 | mega | M |
| 103 | kilo | k |
| 102 | hecto | h |
| 101 | deka | da |
SI prefixes
| Factor | Name | 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
|
It is important to note that the kilogram is the only SI unit with a prefix as part of its name and symbol.
Because multiple prefixes may not be used, in the case of the kilogram the prefix names of Table 5 are used with the unit name "gram" and the prefix symbols are used with the unit symbol "g."
With this exception, any SI prefix may be used with any SI unit, including the degree Celsius and its symbol °C.
Example 1:10-6 kg = 1 mg (one milligram), but not 10-6 kg = 1 µkg (one microkilogram)
Example 2: | Consider the earlier example of the height of the Washington Monument. We may write hW = 169 000 mm = 16 900 cm = 169 m = 0.169 km using the millimeter (SI prefix milli, symbol m), centimeter (SI prefix centi, symbol c), or kilometer (SI prefix kilo, symbol k).
Because the SI prefixes strictly represent powers of 10, they should not be used to represent powers of 2. Thus, one kilobit, or 1 kbit, is 1000 bit and not 210 bit = 1024 bit. To alleviate this ambiguity, prefixes for binary multiples have been adopted by the International Electrotechnical Commission (IEC) for use in information technology.
prefixes
Non SI Units
Units outside the SI
Certain units are not part of the International System of Units, that is, they are outside the SI, but are important and widely used. Consistent with the recommendations of the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures), the units in this category that are accepted for use with the SI are given in Table 6.
Units outside the SI that are accepted for use with the SI
| Name | Symbol | Value in SI units |
|---|
| minute (time) | min | 1 min = 60 s |
| hour | h | 1 h = 60 min = 3600 s |
| day | d | 1 d = 24 h = 86 400 s |
| degree (angle) | ° | 1° = ( pi/180) rad |
| minute (angle) | ' | 1' = (1/60)° = (pi/10 800) rad |
| second (angle) | '' | 1'' = (1/60)' = (pi/648 000) rad |
| liter | L | 1 L = 1 dm3 = 10-3 m3 |
| metric ton (a) | t | 1 t = 103 kg |
| neper | Np | 1 Np = 1 |
| bel (b) | B | 1 B = (1/2) ln 10 Np (c) |
| electronvolt (d) | eV | 1 eV = 1.602 18 x 10-19 J, approximately |
| unified atomic mass unit (e) | u | 1 u = 1.660 54 x 10-27 kg, approximately |
| astronomical unit (f) | ua | 1 ua = 1.495 98 x 1011 m, approximately |
- (a) In many countries, this unit is called "tonne.''
- (b) The bel is most commonly used with the SI prefix deci: 1 dB = 0.1 B.
- (c) Although the neper is coherent with SI units and is accepted by the CIPM, it has not been adopted by the General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures) and is thus not an SI unit.
- (d) The electronvolt is the kinetic energy acquired by an electron passing through a potential difference of 1 V in vacuum. The value must be obtained by experiment, and is therefore not known exactly.
- (e) The unified atomic mass unit is equal to 1/12 of the mass of an unbound atom of the nuclide 12C, at rest and in its ground state. The value must be obtained by experiment, and is therefore not known exactly.
- (f) The astronomical unit is a unit of length. Its value is such that, when used to describe the motion of bodies in the solar system, the heliocentric gravitation constant is (0.017 202 098 95)2 ua3·d-2. The value must be obtained by experiment, and is therefore not known exactly.
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The liter in Table 6 deserves comment. This unit and its symbol l were adopted by the CIPM in 1879. The alternative symbol for the liter, L, was adopted by the CGPM in 1979 in order to avoid the risk of confusion between the letter l and the number 1. Thus, although both l and L are internationally accepted symbols for the liter, to avoid this risk the preferred symbol for use in the United States is L. Neither a lowercase script letter l nor an uppercase script letter L are approved symbols for the liter.
Other units outside the SI that are currently accepted for use with the SI by NIST are given in Table 7. These units, which are subject to future review, should be defined in relation to the SI in every document in which they are used; their continued use is not encouraged. The CIPM currently accepts the use of all of the units given in Table 7 with the SI except for the curie, roentgen, rad, and rem. Because of the continued wide use of these units in the United States, NIST still accepts their use with the SI.
Other units outside the SI that are currently accepted for use with the SI, subject to further review
| Name | Symbol | Value in SI units |
|---|
| nautical mile | | 1 nautical mile = 1852 m |
| knot | | 1 nautical mile per hour = (1852/3600) m/s |
| are | a | 1 a = 1 dam2 = 102 m2 |
| hectare | ha | 1 ha = 1 hm2 = 104 m2 |
| bar | bar | 1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 105 Pa |
| ångström | Å | 1 Å = 0.1 nm = 10-10 m |
| barn | b | 1 b = 100 fm2 = 10-28 m2 |
| curie | Ci | 1 Ci = 3.7 x 1010 Bq |
| roentgen | R | 1 R = 2.58 x 10-4 C/kg |
| rad | rad | 1 rad = 1 cGy = 10-2 Gy |
| rem | rem | 1 rem = 1 cSv = 10-2 Sv |
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