Unit Reference
SI base units and prefixes, derived from the BIPM International System of Units (SI), 9th edition.
Last updated: 2026-01-15
SI Base Units
Seven defining constants of the International System.
| Name | Symbol | Quantity | Definition |
|---|---|---|---|
| second | s | time | Defined by the fixed numerical value of the caesium frequency ΔνCs = 9,192,631,770 Hz. |
| metre | m | length | Defined by the fixed numerical value of the speed of light c = 299,792,458 m/s. |
| kilogram | kg | mass | Defined by the fixed numerical value of the Planck constant h = 6.62607015×10⁻³⁴ J·s. |
| ampere | A | electric current | Defined by the fixed numerical value of the elementary charge e = 1.602176634×10⁻¹⁹ C. |
| kelvin | K | thermodynamic temperature | Defined by the fixed numerical value of the Boltzmann constant k = 1.380649×10⁻²³ J/K. |
| mole | mol | amount of substance | Defined by the fixed numerical value of the Avogadro constant NA = 6.02214076×10²³ mol⁻¹. |
| candela | cd | luminous intensity | Defined by the fixed numerical value of the luminous efficacy Kcd = 683 lm/W. |
SI Prefixes
Decimal multiples and submultiples.
| Name | Symbol | Factor |
|---|---|---|
| quetta | Q | 10³⁰ |
| ronna | R | 10²⁷ |
| yotta | Y | 10²⁴ |
| zetta | Z | 10²¹ |
| exa | E | 10¹⁸ |
| peta | P | 10¹⁵ |
| tera | T | 10¹² |
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| hecto | h | 10² |
| deca | da | 10¹ |
| deci | d | 10⁻¹ |
| centi | c | 10⁻² |
| milli | m | 10⁻³ |
| micro | µ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
| pico | p | 10⁻¹² |
| femto | f | 10⁻¹⁵ |
| atto | a | 10⁻¹⁸ |
| zepto | z | 10⁻²¹ |
| yocto | y | 10⁻²⁴ |
| ronto | r | 10⁻²⁷ |
| quecto | q | 10⁻³⁰ |
Source: Bureau International des Poids et Mesures (BIPM), The International System of Units (SI), 9th edition, 2019. https://www.bipm.org/en/publications/si-brochure