SI Unit Prefixes
Twenty prefixes are used by the SI sytem of measurement to form decimal multiples and submultiples of the base and derived SI units.
See our page on
SI Units for more information on the base and derived units.
Many of these prefixes are in every day use in engineering reports, control system displays etc. e.g. Megawatt, kilometer. Care has to be taken when using engineering calculations as they often require the base or derived unit to be used, e.g. instead of 3 Megawatts formulae often require 3,000,000 watts to be used.
| 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 |
| 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 |
Points to Note
- The kilogram is the only SI unit with a prefix as part of its name and symbol i.e. kg. Under the SI convention multiple prefixes may not be used, therefore when using kilogram the prefix names above are used with the unit name "gram" and the prefix symbols are used with the unit symbol "g."
- Apart from the kilogram, any SI prefix may be used with any SI unit, including the degree Celsius and its symbol °C. Note however, it is unusual to use prefixes with Celsius.
- The SI prefixes represent powers of 10, therefore their use for binary multiples e.g. one kilobit, or 1 kbit, which is actually 1024 bits is not strictly correct. However, the International Electrotechnical Commission (IEC) has adopted this system of prefixing for use in information technology.
