Reckoning / Measure / Prefixes
Measure · 24 prefixes
Prefixes
A prefix multiplies a unit by a power of ten. Above a thousand they step by a thousand; the four newest, ronna, quetta, ronto and quecto, were added in 2022, mainly because data storage and the mass of planets needed them.
| Symbol | Prefix | Factor | Written out | Adopted |
|---|---|---|---|---|
| Q | quetta | 1030 | 1 000 000 000 000 000 000 000 000 000 000 | 2022 new |
| R | ronna | 1027 | 1 000 000 000 000 000 000 000 000 000 | 2022 new |
| Y | yotta | 1024 | 1 000 000 000 000 000 000 000 000 | 1991 |
| Z | zetta | 1021 | 1 000 000 000 000 000 000 000 | 1991 |
| E | exa | 1018 | 1 000 000 000 000 000 000 | 1975 |
| P | peta | 1015 | 1 000 000 000 000 000 | 1975 |
| T | tera | 1012 | 1 000 000 000 000 | 1960 |
| G | giga | 109 | 1 000 000 000 | 1960 |
| M | mega | 106 | 1 000 000 | 1960 |
| k | kilo | 103 | 1 000 | 1795 |
| h | hecto | 102 | 100 | 1795 |
| da | deca | 101 | 10 | 1795 |
| d | deci | 10-1 | 0.1 | 1795 |
| c | centi | 10-2 | 0.01 | 1795 |
| m | milli | 10-3 | 0.001 | 1795 |
| µ | micro | 10-6 | 0.000 001 | 1960 |
| n | nano | 10-9 | 0.000 000 001 | 1960 |
| p | pico | 10-12 | 0.000 000 000 001 | 1960 |
| f | femto | 10-15 | 0.000 000 000 000 001 | 1964 |
| a | atto | 10-18 | 0.000 000 000 000 000 001 | 1964 |
| z | zepto | 10-21 | 0.000 000 000 000 000 000 001 | 1991 |
| y | yocto | 10-24 | 0.000 000 000 000 000 000 000 001 | 1991 |
| r | ronto | 10-27 | 0.000 000 000 000 000 000 000 000 001 | 2022 new |
| q | quecto | 10-30 | 0.000 000 000 000 000 000 000 000 000 001 | 2022 new |
Sources: The International System of Units (SI), 9th edition (2019), with later updates; Resolution 3 of the 27th General Conference on Weights and Measures (2022): ronna, quetta, ronto, quecto.
Then and now
Tables printed before 1991 stop at exa and atto, and those before 2022 at yotta and yocto. Never combine prefixes: write nanometre, not millimicrometre. For mass, prefixes attach to the gram: milligram, not microkilogram.