SI prefixes
| Prefix | Symbol | Multiplier |
| giga | G | 109 |
| mega | M | 106 |
| kilo | k | 103 |
| milli | m | 10-3 |
| micro | ? | 10-6 |
| nano | n | 10-9 |
| pico | p | 10-12 |
| femto | f | 10-15 |
This is a summary of the most commonly used SI prefixes, e.g.:
4.782 km = 4782 m = 7.782×103 m
SI units
| Base quantity | Base unit | ||
| time | t | second | s |
| length | l | metre | m |
| mass | m | kilogram | kg |
| temperature | T | kelvin | K |
| electric current | I | ampere | A |
| amount of substance | n | mole | mol |
| luminous intensity | Iv | candela | cd |
There are seven base units from which all other units can be derived. For example, the units for speed can be given as ms-1.
Some derived quantities have been assigned a special named unit, e.g.:
| Derived quantity | Derived unit | |||
| force | F | kg·m·s-1 | newton | N |
| pressure | P | kg·m·s-2m-2 = N·m-2 | pascal | Pa |
| energy | E | kg·m·s-2m = N·m | joule | J |
| power | P | kg·m·s-2m·s-1 = J·s-1 | watt | W |
| electrical charge | Q | A·s | coulomb | C |
| potential difference | V | kg·m·s-2m·A-1·s-1 = J·C-1 | volt | V |
| resistance | r | kg·m·s-2m·A-1·s-1A-1 = V·A-1 | ohm | ? |
Homogeneity
The units of an equation must be consistent on both sides, just as the values must. This is called being homogeneous.
To check for homogeneity, reduce all derived units to base units and the simplify. The units left should be identical on both sides, or the equation is wrong.