Metric Prefixes Chart
All 24 SI prefixes, from quecto to quetta. Most charts still stop at yocto and yotta and are four prefixes out of date.
The full chart
| Prefix | Symbol | Factor | Decimal |
|---|---|---|---|
| quetta | Q | 1030 | 1 000 000 000 000 000 000 000 000 000 000 |
| ronna | R | 1027 | 1 000 000 000 000 000 000 000 000 000 |
| 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 |
| deka | 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 | 0.000 001 |
| nano | n | 10-9 | 0.000 000 001 |
| pico | p | 10-12 | 0.000 000 000 001 |
| femto | f | 10-15 | 0.000 000 000 000 001 |
| atto | a | 10-18 | 0.000 000 000 000 000 001 |
| zepto | z | 10-21 | 0.000 000 000 000 000 000 001 |
| yocto | y | 10-24 | 0.000 000 000 000 000 000 000 001 |
| ronto | r | 10-27 | 0.000 000 000 000 000 000 000 000 001 |
| quecto | q | 10-30 | 0.000 000 000 000 000 000 000 000 000 001 |
The four added in 2022
On 18 November 2022 the 27th General Conference on Weights and Measures approved four new prefixes, the first expansion of the range since 1991. Ronna (R) covers 1027 and quetta (Q) covers 1030 at the large end. Ronto (r) covers 10-27 and quecto (q) covers 10-30 at the small end. That took the SI from 20 prefixes to 24.
The change was driven largely by data storage, which was already using the top of the old range in zettabytes and yottabytes, and by the need to keep the small end symmetrical with the large one. The letters R and Q were chosen because they were the last two not already in use.
Any prefix chart that ends at yotta and yocto was written before this and is incomplete rather than wrong.
Capitalization rules that trip people up
Prefix symbols for submultiples are always lowercase. Prefix symbols for multiples are uppercase, with three exceptions: kilo is a lowercase k, hecto is a lowercase h, and deka is a lowercase da.
The practical consequences are worth memorizing. A capital M means mega and a lowercase m means milli, a difference of a billion. A capital K isn't a valid SI prefix symbol at all; kilo is always lowercase. And a capital N is the newton while a lowercase n is nano, which is the ambiguity behind one of the entries on the common mistakes page.
Why kibi and mebi aren't on this chart
Kibi, mebi and gibi are binary prefixes, powers of 1,024 rather than powers of 1,000. They are defined by the IEC rather than being part of the SI, which is why they don't appear above.
The confusion between the two systems is why a 1 TB drive shows as roughly 931 GB once formatted. That is covered on the common mistakes page.
Common questions
How many SI prefixes are there?
Twenty-four, since November 2022. Ronna, quetta, ronto and quecto were added at the 27th General Conference on Weights and Measures, the first expansion since 1991.
What is the largest SI prefix?
Quetta, symbol Q, meaning 10 to the power 30. The smallest is quecto, symbol q, at 10 to the power minus 30.
Why is kilo written with a lowercase k?
Kilo, hecto and deka are the three exceptions to the rule that multiple prefixes take an uppercase symbol. A capital K isn't a valid SI prefix symbol.
Are kibi, mebi and gibi SI prefixes?
No. Those are binary prefixes defined by the IEC, based on powers of 1,024 rather than 1,000. They aren't part of the SI, which is why they don't appear in the chart above.
Related guides
Sources
- Extension of the range of SI prefixes. International Bureau of Weights and Measures (BIPM), 19 December 2022. Primary source for the four prefixes approved on 18 November 2022 and their symbols.
- SI Prefix Progress. National Institute of Standards and Technology. Source for the expansion from 20 to 24 prefixes and for the capitalization rules, including the kilo, hecto and deka exceptions.
- NIST Guide to the SI, Appendix B. Source for the factor values in the chart.