SI prefixes — the official table
The International System of Units defines exactly 24 prefixes, specified in the BIPM SI Brochure (9th edition; range extended to quetta/quecto in 2022). Compound prefixes such as “kilomega” are not permitted.
| Name | Symbol | Factor | As decimal |
|---|---|---|---|
| quetta | Q | 10³⁰ | None |
| ronna | R | 10²⁷ | None |
| yotta | Y | 10²⁴ | 1,000,000,000,000,000,000,000,000 |
| zetta | Z | 10²¹ | 1,000,000,000,000,000,000,000 |
| exa | E | 10¹⁸ | 1,000,000,000,000,000,000 |
| peta | P | 10¹⁵ | 1,000,000,000,000,000 |
| tera | T | 10¹² | 1,000,000,000,000 |
| giga | G | 10⁹ | 1,000,000,000 |
| mega | M | 10⁶ | 1,000,000 |
| kilo | k | 10³ | 1,000 |
| hecto | h | 10² | 100 |
| deca | da | 10¹ | 10 |
| deci | d | 10⁻¹ | 0.1 |
| centi | c | 10⁻² | 0.01 |
| milli | m | 10⁻³ | 0.001 |
| micro | µ | 10⁻⁶ | 0.000001 |
| nano | n | 10⁻⁹ | 0.000000001 |
| pico | p | 10⁻¹² | 0.000000000001 |
| femto | f | 10⁻¹⁵ | 0.000000000000001 |
| atto | a | 10⁻¹⁸ | 0.000000000000000001 |
| zepto | z | 10⁻²¹ | — |
| yocto | y | 10⁻²⁴ | — |
| ronto | r | 10⁻²⁷ | — |
| quecto | q | 10⁻³⁰ | — |
Rules worth knowing
- Symbols for factors ≥ 10⁶ are uppercase (M, G, T …); below that lowercase — kilo is
k, not K. - The prefix attaches to the unit symbol without a gap:
mm, notm m. - Binary prefixes (KiB = 2¹⁰ bytes) are separate IEC prefixes — see GiB ↔ GB.
Source: BIPM, The International System of Units (SI Brochure), 9th ed.; CGPM 2022 extension. This page restates the definitions; the brochure remains authoritative.