Science & Units
SI Prefix Reference
An SI prefix is a shorthand for a power of ten. It is the reason a nanometre and a kilometre can be compared without writing out nine zeros, and the reason unit conversion is mostly a matter of counting decimal places rather than multiplying by awkward factors. Once you know that nano means 10⁻⁹ and kilo means 10³, a whole family of conversions collapses into moving a decimal point.
The system is deliberately regular: each prefix is a single syllable, each symbol is a single letter, and prefixes above 10³ step by thousands. The letters are case-sensitive, and the case carries meaning — mm is a millimetre, Mm is a megametre, a factor of a billion apart. This is the single most common source of unit errors in engineering documents.
In 2022 the 27th General Conference on Weights and Measures added the four outermost prefixes — quecto, ronto, ronna and quetta — because data volumes and astronomical measurements had run out of room at both ends. The binary prefixes in the second table are a separate system, defined by IEC rather than SI, and exist because computer memory is addressed in powers of two.
SI decimal prefixes
| Prefix | Symbol | Power | Decimal value | Where you meet it |
|---|---|---|---|---|
| quecto | q | 10⁻³⁰ | 0.000000000000000000000000000001 | Mass of a single electron is about 0.001 qg |
| ronto | r | 10⁻²⁷ | 0.000000000000000000000000001 | Used for subatomic scales below yocto |
| yocto | y | 10⁻²⁴ | 0.000000000000000000000001 | Mass of a proton is about 1.67 yg |
| zepto | z | 10⁻²¹ | 0.000000000000000000001 | Charge on an electron is 160 zC |
| atto | a | 10⁻¹⁸ | 0.000000000000000001 | Attosecond physics won the 2023 Nobel Prize |
| femto | f | 10⁻¹⁵ | 0.000000000000001 | Light travels 0.3 µm in one femtosecond |
| pico | p | 10⁻¹² | 0.000000000001 | A pF capacitor, a picometre atomic radius |
| nano | n | 10⁻⁹ | 0.000000001 | A 3 nm chip process, a 100 nm virus |
| micro | µ | 10⁻⁶ | 0.000001 | A 20 µF capacitor, a 5 µm pollen grain |
| milli | m | 10⁻³ | 0.001 | A 500 mg tablet, a 2 mm pencil lead |
| centi | c | 10⁻² | 0.01 | A centimetre, a centilitre of wine |
| deci | d | 10⁻¹ | 0.1 | A decibel, a decilitre of milk |
| (none) | — | 10⁰ | 1 | The base unit: metre, gram, litre, second |
| deca | da | 10¹ | 10 | A decametre, rarely used outside surveying |
| hecto | h | 10² | 100 | A hectare is a square hectometre |
| kilo | k | 10³ | 1000 | A kilometre, a kilogram, a kilowatt-hour |
| mega | M | 10⁶ | 1000000 | A megawatt, a megabyte, a megahertz |
| giga | G | 10⁹ | 1000000000 | A gigabyte, a gigahertz, a gigawatt |
| tera | T | 10¹² | 1000000000000 | A terabyte, a terawatt of solar capacity |
| peta | P | 10¹⁵ | 1000000000000000 | A petabyte of storage, a petaflop of compute |
| exa | E | 10¹⁸ | 1000000000000000000 | An exabyte of global internet traffic |
| zetta | Z | 10²¹ | 1000000000000000000000 | Zettabytes of data created each year |
| yotta | Y | 10²⁴ | 1000000000000000000000000 | A yottabyte, the upper end of SI prefixes until 2022 |
| ronna | R | 10²⁷ | 1000000000000000000000000000 | Added in 2022 for data and astronomy scales |
| quetta | Q | 10³⁰ | 1000000000000000000000000000000 | Mass of the Earth is about 6 Qg |
Quecto, ronto, ronna and quetta were added by the 27th CGPM in 2022. Micro is the only prefix written with a Greek letter.
Binary prefixes (IEC 80000-13)
| Prefix | Symbol | Power of 2 | Value | Closest decimal prefix |
|---|---|---|---|---|
| kibi | Ki | 2¹⁰ | 1,024 | 2.4% above kilo |
| mebi | Mi | 2²⁰ | 1,048,576 | 4.9% above mega |
| gibi | Gi | 2³⁰ | 1,073,741,824 | 7.4% above giga |
| tebi | Ti | 2⁴⁰ | 1,099,511,627,776 | 10% above tera |
| pebi | Pi | 2⁵⁰ | 1,125,899,906,842,624 | 12.6% above peta |
A kibibyte is 1024 bytes; a kilobyte is 1000. The 2.4% gap compounds to 10% by the tebibyte.
The seven SI base units
| Quantity | Unit | Symbol | Defined by |
|---|---|---|---|
| Time | second | s | The caesium-133 hyperfine transition frequency, 9 192 631 770 Hz |
| Length | metre | m | The speed of light in vacuum, 299 792 458 m/s |
| Mass | kilogram | kg | The Planck constant, 6.626 070 15 × 10⁻³⁴ J·s |
| Electric current | ampere | A | The elementary charge, 1.602 176 634 × 10⁻¹⁹ C |
| Temperature | kelvin | K | The Boltzmann constant, 1.380 649 × 10⁻²³ J/K |
| Amount of substance | mole | mol | The Avogadro constant, 6.022 140 76 × 10²³ mol⁻¹ |
| Luminous intensity | candela | cd | The luminous efficacy of 540 THz radiation, 683 lm/W |
Since the 2019 redefinition, every base unit is fixed by a physical constant rather than by a physical artefact.
How to Use These Tables
- To convert between prefixes, subtract the exponents. Kilometres to millimetres is 10³ to 10⁻³, a difference of 6, so multiply by 10⁶.
- Watch the case. m is milli and M is mega; n is nano and N is newton. A capital letter is never a stylistic choice in a unit symbol.
- For volumes, remember that prefixes apply to the linear unit: 1 km³ is 10⁹ m³, not 10³ m³, because the prefix is cubed along with the unit.
- Do not mix binary and decimal prefixes in one calculation. Pick KiB or kB and stay there — this is exactly the confusion behind the missing-capacity complaint on hard drives.
Common Questions
What is smaller than a nano?
Pico (10⁻¹²), femto (10⁻¹⁵), atto (10⁻¹⁸), zepto (10⁻²¹), yocto (10⁻²⁴), ronto (10⁻²⁷) and quecto (10⁻³⁰), in that order. Each step is a factor of a thousand.
Why is kilo lowercase but mega uppercase?
Because the symbols were fixed historically: kilo, hecto and deca got lowercase letters, and every prefix from mega upward got uppercase. The rule is mechanical, not phonetic, which is why mega is M and milli is m.
Is a microgram written µg or mcg?
Both appear. µg is the SI form; mcg is used in medicine and on supplement labels to avoid confusion with mg when handwriting is unclear. They mean the same thing — one millionth of a gram.
What is the difference between a megabyte and a mebibyte?
A megabyte is 1 000 000 bytes; a mebibyte is 1 048 576 bytes, about 4.9% more. Storage manufacturers use megabytes, operating systems traditionally report mebibytes while labelling them MB.
Are SI prefixes used with non-SI units?
Yes, widely — milliseconds, kilowatt-hours, microsieverts. The prefix attaches to the unit symbol, and the unit itself does not change: a kilowatt-hour is a unit of energy, not a unit of power.
Sources
- The International System of Units (SI Brochure), 9th edition — Table 7, SI prefixes — Bureau International des Poids et Mesures
- NIST Special Publication 811 — Guide for the Use of the International System of Units — National Institute of Standards and Technology
- ISO/IEC 80000-13 — Information science and technology quantities and units — International Organization for Standardization
Filed with Science calculations.