SI Prefixes
SI prefixes are multipliers that scale a unit by powers of ten, letting us write very large or very small quantities compactly — a kilometre is 1000 metres, a millisecond is one thousandth of a second.
The 24 SI prefixes
There are 24 prefixes spanning 10³⁰ down to 10⁻³⁰. The symbol is attached directly to the unit symbol (e.g. km, µg, GHz).
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| quetta 2022 | Q | 1030 | Qm |
| ronna 2022 | R | 1027 | Rm |
| yotta | Y | 1024 | Ym |
| zetta | Z | 1021 | Zm |
| exa | E | 1018 | Em |
| peta | P | 1015 | Pm |
| tera | T | 1012 | Tm |
| giga | G | 109 | Gm |
| mega | M | 106 | Mm |
| kilo | k | 103 | km |
| hecto | h | 102 | hm |
| deca | da | 101 | dam |
| deci | d | 10-1 | dm |
| centi | c | 10-2 | cm |
| milli | m | 10-3 | mm |
| micro | µ | 10-6 | µm |
| nano | n | 10-9 | nm |
| pico | p | 10-12 | pm |
| femto | f | 10-15 | fm |
| atto | a | 10-18 | am |
| zepto | z | 10-21 | zm |
| yocto | y | 10-24 | ym |
| ronto 2022 | r | 10-27 | rm |
| quecto 2022 | q | 10-30 | qm |
The 2022 additions
In 2022 the General Conference on Weights and Measures (CGPM) added four new prefixes — ronna (10²⁷), quetta (10³⁰), ronto (10⁻²⁷) and quecto (10⁻³⁰) — largely to express the huge data volumes and tiny masses of modern science.
How to use prefixes
Use only one prefix at a time (no “kilokilogram”), and remember that for mass the base unit is the kilogram, so prefixes attach to the gram (e.g. milligram, not microkilogram).
Frequently asked questions
What is the largest SI prefix?
Quetta (Q), equal to 10³⁰, is the largest SI prefix, added in 2022 along with ronna (10²⁷).
What is the smallest SI prefix?
Quecto (q), equal to 10⁻³⁰, is the smallest SI prefix, added in 2022 along with ronto (10⁻²⁷).
Why is the symbol for micro “µ”?
The micro prefix (10⁻⁶) uses the Greek letter mu (µ). Where the character is unavailable it is often written as “u”, e.g. “um” for micrometre.