Base Units
The SI system of units has 7 base units, from which all other units are derived:
| Quantity | Unit | abbreviation |
|---|---|---|
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
| temperature | kelvin | K |
| electrical current | ampere | A |
| amount of a substance | mole | mol |
| luminous intensity | candela | cd |
Note: Luminous intensity is not generally covered at GCSE or A level.
| Prefix | Base 10 | Decimal | English word | ||
|---|---|---|---|---|---|
| Name | Symbol | Short scale | Long scale | ||
| yotta | Y | 1024 | 1 000 000 000 000 000 000 000 000 | septillion | quadrillion |
| zetta | Z | 1021 | 1 000 000 000 000 000 000 000 | sextillion | trilliard |
| exa | E | 1018 | 1 000 000 000 000 000 000 | quintillion | trillion |
| peta | P | 1015 | 1 000 000 000 000 000 | quadrillion | billiard |
| tera | T | 1012 | 1 000 000 000 000 | trillion | billion |
| giga | G | 109 | 1 000 000 000 | billion | milliard |
| mega | M | 106 | 1 000 000 | million | |
| kilo | k | 103 | 1 000 | thousand | |
| hecto | h | 102 | 100 | hundred | |
| deca | da | 101 | 10 | ten | |
| 100 | 1 | one | |||
| deci | d | 10-1 | 0.1 | tenth | |
| centi | c | 10-2 | 0.01 | hundredth | |
| milli | m | 10-3 | 0.001 | thousandth | |
| micro | µ | 10-6 | 0.000 001 | millionth | |
| nano | n | 10-9 | 0.000 000 001 | billionth | milliardth |
| pico | p | 10-12 | 0.000 000 000 001 | trillionth | billionth |
| femto | f | 10-15 | 0.000 000 000 000 001 | quadrillionth | billiardth |
| atto | a | 10-18 | 0.000 000 000 000 000 001 | quintillionth | trillionth |
| zepto | z | 10-21 | 0.000 000 000 000 000 000 001 | sextillionth | trilliardth |
| yocto | y | 10-24 | 0.000 000 000 000 000 000 000 001 | septillionth | quadrillionth |
All the prefixes are listed here, but only those between tera and femto are needed for GCSE and A level.