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# SI prefix

This article has been reviewed by the following Topic Editor: Cutler J. Cleveland
The International Electrotechnical Commission established the prefix system in 1998.

One of the advantages of the International System of Units is its combination of metric terminology with its decimal organization. There are prefixes associated with a decimal position and can be attached to the base SI unit in order to create a new unit. The knowledge of the decimal meaning of the prefix establishes the relationship between the newly created unit and the base unit.

The twenty SI prefixes used to form decimal multiples and submultiples of SI units are given below.

SI prefixes
10n Prefix Symbol Name Decimal equivalent in SI
1024 yotta Y Septillion 1 000 000 000 000 000 000 000 000
1021 zetta Z Sextillion 1 000 000 000 000 000 000 000
1018 exa E Quintillion 1 000 000 000 000 000 000
1015 peta P Quadrillion 1 000 000 000 000 000
1012 tera T Trillion 1 000 000 000 000
109 giga G Billion 1 000 000 000
106 mega M Million 1 000 000
103 kilo k Thousand 1 000
102 hecto h Hundred 100
101 deca, deka da Ten 10
100 (none) (none) One 1
10?1 deci d Tenth 0.1
10?2 centi c Hundredth 0.01
10?3 milli m Thousandth 0.001
10?6 micro µ (u) Millionth 0.000 001
10?9 nano n Billionth 0.000 000 001
10?12 pico p Trillionth 0.000 000 000 001
10?15 femto f Quadrillionth 0.000 000 000 000 001
10?18 atto a Quintillionth 0.000 000 000 000 000 001
10?21 zepto z Sextillionth 0.000 000 000 000 000 000 001
10?24 yocto y Septillionth 0.000 000 000 000 000 000 000 001

The are specific rules to be observed when using the SI prefixes:

1. Prefix symbols are printed in roman (upright) type with no space between the prefix symbol and the unit symbol. For example, one million joules (J) would be designated MJ.
2. The grouping formed by the prefix symbol attached to the unit symbol constitutes a new inseparable symbol (of a multiple or submultiple of the unit concerned) which can be raised to a positive or negative power and combined with other unit symbols to form compound unit symbols.
Examples:
1 cm3 = (10–2 m)3 = 10–6 m3
</dd>
1 µs–1 = (10–6 s)–1 = 106 s–1
</dd>
1 V/cm = (1 V)/(10–2 m) = 102 V/m
</dd>
1 cm–1 = (10–2 m)–1 = 102 m–1
</dd>
</dd>
3. Compound prefixes, i.e. prefixes formed by the juxtaposition of two or more SI prefixes, are not used.
Example: 1 nm but not 1 mµm
</dd>
4. A prefix is never used in isolation.
Example: 106/m–3 but not M/m–3. </dd>

Other tips for using prefixes:

• A very common mistake is that the prefix milli- stands for a millionth. As can be seen from the table above, milli stands for a thousandth. It comes from the French, mille for 1000—they could not use it for the 1000 prefix as that was taken by the Greek word, kilo.
• Don't confuse scientific notation with powers. A million is 10 to the power of 6 (106), but if you confused it with the scientific notation and had 17 the answer would be 1 and not a million (you say one times one is one, seven times, and the answer is still one).
• The kilogram is unique among all SI base units becasue it is the only one that has a prefix. It is derived from the mass of an actual object. The gram is defined as 1/1000 of this object's mass.
• The SI system is different that the binary prefix system that is used in computing, where they are applied to information and storage units like the bit and the byte.

 Disclaimer: This article is taken wholly from, or contains information that was originally published by, the National Institue of Standards and Technology. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the National Institue of Standards and Technology should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

## Citation

NIST (Content Source);Cutler J. Cleveland (Topic Editor) "SI prefix". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth September 2, 2008; Last revised Date September 2, 2008; Retrieved May 24, 2013 <http://www.eoearth.org/article/SI_prefix>