SI prefixes
The complete set of metric prefixes, from quetta down to quecto, plus the IEC binary prefixes used for digital storage. Every prefix is a fixed power of ten, so converting between them only ever moves the decimal point.
Decimal (SI) prefixes
| Prefix | Symbol | Factor | Decimal | Adopted | Example |
|---|---|---|---|---|---|
| Quetta | Q | 10³⁰ | 1,000,000,000,000,000,000,000,000,000,000 | 2022 | — |
| Ronna | R | 10²⁷ | 1,000,000,000,000,000,000,000,000,000 | 2022 | — |
| Yotta | Y | 10²⁴ | 1,000,000,000,000,000,000,000,000 | 1991 | yottabyte (YB) |
| Zetta | Z | 10²¹ | 1,000,000,000,000,000,000,000 | 1991 | zettabyte (ZB) |
| Exa | E | 10¹⁸ | 1,000,000,000,000,000,000 | 1975 | exameter (Em) |
| Peta | P | 10¹⁵ | 1,000,000,000,000,000 | 1975 | petabyte (PB) |
| Tera | T | 10¹² | 1,000,000,000,000 | 1960 | terahertz (THz) |
| Giga | G | 10⁹ | 1,000,000,000 | 1960 | gigawatt (GW) |
| Mega | M | 10⁶ | 1,000,000 | 1873 | megapascal (MPa) |
| Kilo | k | 10³ | 1,000 | 1795 | kilometer (km) |
| Hecto | h | 10² | 100 | 1795 | hectopascal (hPa) |
| Deka | da | 10¹ | 10 | 1795 | dekameter (dam) |
| Deci | d | 10⁻¹ | 0.1 | 1795 | deciliter (dL) |
| Centi | c | 10⁻² | 0.01 | 1795 | centimeter (cm) |
| Milli | m | 10⁻³ | 0.001 | 1795 | milligram (mg) |
| Micro | µ | 10⁻⁶ | 0.000001 | 1873 | micrometer (µm) |
| Nano | n | 10⁻⁹ | 0.000000001 | 1960 | nanosecond (ns) |
| Pico | p | 10⁻¹² | 0.000000000001 | 1960 | picofarad (pF) |
| Femto | f | 10⁻¹⁵ | 0.000000000000001 | 1964 | femtosecond (fs) |
| Atto | a | 10⁻¹⁸ | 0.000000000000000001 | 1964 | attojoule (aJ) |
| Zepto | z | 10⁻²¹ | 0.000000000000000000001 | 1991 | — |
| Yocto | y | 10⁻²⁴ | 0.000000000000000000000001 | 1991 | — |
| Ronto | r | 10⁻²⁷ | 0.000000000000000000000000001 | 2022 | — |
| Quecto | q | 10⁻³⁰ | 0.000000000000000000000000000001 | 2022 | — |
Binary (IEC) prefixes
Defined by IEC 60027-2 to remove the ambiguity in using SI prefixes for powers of 1024. Operating systems generally report binary units while drive manufacturers label capacity in decimal units.
| Prefix | Symbol | Factor | Bytes | vs SI prefix |
|---|---|---|---|---|
| Kibi | Ki | 2¹⁰ | 1,024 | +2.4% |
| Mebi | Mi | 2²⁰ | 1,048,576 | +4.9% |
| Gibi | Gi | 2³⁰ | 1,073,741,824 | +7.4% |
| Tebi | Ti | 2⁴⁰ | 1,099,511,627,776 | +10.0% |
| Pebi | Pi | 2⁵⁰ | 1,125,899,906,842,624 | +12.6% |
| Exbi | Ei | 2⁶⁰ | 1,152,921,504,606,846,976 | +15.3% |
| Zebi | Zi | 2⁷⁰ | 1,180,591,620,717,411,303,424 | +18.1% |
| Yobi | Yi | 2⁸⁰ | 1,208,925,819,614,629,174,706,176 | +20.9% |
Need to convert actual file sizes? Use the data storage converter.
How to use prefixes correctly
- Case matters.
mis milli,Mis mega — a factor of a billion apart. - One prefix at a time. Write nanometre, not millimicrometre.
- Attach the prefix to the unit, not the number. 5 km, never 5k m.
- Prefixes bind tightly. km² means (km)², an area of 10⁶ m², not 1,000 m².
- The kilogram is the exception. It is the SI base unit for mass but already carries a prefix, so multiples are built on the gram: milligram, not microkilogram.
Frequently asked questions
What is the largest SI prefix?
Quetta (Q), meaning 10³⁰. It was adopted along with ronna, ronto, and quecto at the 27th General Conference on Weights and Measures in November 2022, largely because global data volumes were approaching the limits of yotta.
Why do KB and KiB mean different things?
SI prefixes are powers of 1000, so a kilobyte is 1,000 bytes. The IEC binary prefixes are powers of 1024, so a kibibyte is 1,024 bytes. The gap compounds: a terabyte is about 9% smaller than a tebibyte, which is why a 1 TB drive shows as roughly 931 GiB.
Is it kilo with a lowercase or uppercase k?
Lowercase. Every prefix below mega uses a lowercase symbol, and every prefix from mega upward uses an uppercase one. Writing KM for kilometre is incorrect — K is not an SI prefix symbol at all.
Can prefixes be stacked?
No. Compound prefixes such as millimicrometre are not allowed; use the single prefix that fits, in this case nanometre. The kilogram is the one oddity — it is the SI base unit despite already carrying a prefix, so multiples are formed from the gram.