# SI Units

“Systeme International”, or

**SI Units**, is a standardised system of measurement based on internationally agreed definitions.- It has 7
**Base Units**, from which all other units are derived:**Metre (m)**- Length**Kilogram (Kg)**- Mass**Second (s)**- Time**Kelvin (°K)**- temperature**Ampere (A)**- Current**Candela (Cd)**- Luminous Intensity**Mole (mol)**- Amount of a substance

All the base units have standard definition, for example, the metre is the distance travelled by light in a vacuum in seconds.

- Other units are derived from the base units. For example:
**Velocity (ms**^{-1})**Acceleration (ms**^{-2})**Density (Kgm**^{-2})

- Some of the derived units also have their own special unit names. For example:
**Coulomb (C)**- (As) Charge**Pascal (Pa)**- (Kgm^{-1}s^{-2}) Pressure**Ohms (Ω)**- (Kgm^{2}s^{-3}A^{-2}) Resistence

## Prefixes

- In order to make working with large or small numbers more convenient, a system of
**prefixes**is used, where the unit is multiplied by a certain**power of ten**:**Yotta (Y)**- 10^{24}**Zetta (Z)**- 10^{21}**Exa (E)**- 10^{18}**Peta (P)**- 10^{15}**Tera (T)**- 10^{12}**Giga (G)**- 10^{9}**Mega (M)**- 10^{6}**Kilo (k)**- 10^{3}**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}

- In addition, these prefixes are also used:
**Hecto (h)**- 10^{2}**Deca (da)**- 10^{1}**Deci (d)**- 10-^{-1}**Centi (c)**- 10^{-2}

*For example, 0.005 A can be written as 5 mA, and 6900000000000000000000000 m can be written as 6.9 Ym.*