The SI system of units (Système International d'Unités) should be used for measurements. The SI system is completely coherent, which means that all derived units are formed by simple multiplication or division of base units without the need for any numerical factors or powers of ten. This distinguishes the SI system from earlier metric systems such as the centimetre-gramme-second (CGS) system, which it superseded. The SI system comprises nine base units, each of which is independently defined, and various other units which are derived by combining two or more base units. The base units, together with some of the more common derived units, are listed in Table A2.1. Some common non-SI units and their SI equivalents are shown in Table A2.2.

Conventions. Each unit is represented by a standard unit symbol (e.g., m, s, A, kg), which may be multiplied or divided by other unit symbols or numbers (e.g., 3 m, 0.12 kg m, 16.5 m s-2). Unit symbols are algebraic symbols and follow the conventions of algebra. They are not abbreviations, and should never be followed by a full stop or an ‘s’ (to denote plural). The names of units (e.g., metre, second, ampere) are all spelt with a lower case initial letter. Symbols for units named after a person start with an upper case letter (e.g., A for ampere, K for kelvin, Pa for pascal). When a measurement is used as an adjective, the number and unit should be joined by a hyphen (e.g., three-metre tube, 5-m distance, 9-s delay, 6-m2 area, seventy-kilogram adult, 2-A current).