Conventions for the use of SI Units

SI lays down precise conventions for how unit symbols must be written, so that scientific writing is unambiguous everywhere in the world:

1. The unit of every physical quantity should be represented by its symbol (not spelled out) once a value has been introduced.

2. The full name of a unit always starts with a lower-case letter, even when the unit is named after a person — e.g. '1 newton', '1 joule'. But the symbol for a unit named after a person is capitalised — e.g. N (after Newton), J (after Joule).

3. Unit symbols never take a plural form: a force of 20 N is written '20 N', never '20 newtons' or '20 Ns'.

4. Unit symbols never carry a full stop at the end (except at the natural end of a sentence): '25 kg', never '25 kg.'.

5. When a unit involves both a numerator and a denominator, it should be written as a single combined ratio rather than repeated division — the SI unit of acceleration is written $\text{m/s}^2$ or $\text{m s}^{-2}$, never $\text{m/s/s}$.

6. A combination of a written-out unit name and a unit symbol should be avoided when a physical quantity is expressed as a combination of two units — e.g. $\text{J/kg K}$ is correct, but 'joule/kg K' is not. …