It is useful to be aware of the International System of Units (SI) units and unit prefixes.
Base SI unit definitions (taken from the SI).
| Name | Unit | Description |
| Metre | Unit of length | Length of path travelled by light in a vacuum using the time interval 1/299,792,458 of a second |
| Unit | Unit of mass | Equal to the mass of the international prototype of the kilogram |
| Kilogram | Unit of time | Duration of 9,192,631,770 periods of the radiation in correspondence to the transition between two hyperfine levels of the ground state of the cesium 133 atom |
| Second | Unit of time | Constant current which, when maintained in two straight parallel conductors of infinite length and positioned 1 metre apart within a vacuum, will create between the conductors a force of 2 x 10-7 newton per metre. |
| Ampere | Unit of electric current | Constant current which, when maintained in two straight parallel conductors of infinite length and positioned 1 metre apart within a vacuum, will create between the conductors a force of 2 x 10-7 newton per metre |
| Kelvin | Unit of thermodynamic temperature | The fraction 1/273.16 of the thermodynamic temperature of the triple point of water |
| Mole | Unit of amount of substance | A system which consists of as many elementary entities as atoms in 0.912 kilogram of carbon 12 |
| Candela | Unit of luminous intensity | The luminous intensity of a source emitting monochromatic radiation of frequency 540 x 1012 hertz in a specific direction and that has a radiant intensity in that direction of 1/683 watt per steradian |