SI units and their use

When describing a measurement, you normally state both a number and a unit (e.g. 'the length is 1.85 metres'). The number expresses the ratio of the measured quantity to a fixed standard, while the unit identifies that standard measure or dimension. Clearly, a single unified system of units is essential for efficient communication of such data within the scientific community. The Systeme International d'Unites (SI) is the internationally ratified form of the metre-kilogram-second system of measurement and represents the accepted scientific convention for measurements of physical quantities.

Another important reason for adopting consistent units is to simplify complex calculations where you may be dealing with several measured quantities. Although the rules of the SI are complex and the scale of the base units is sometimes inconvenient, to gain the full benefits of the system you should observe its conventions strictly.
The description of measurements in SI involves:

• seven base units and two supplementary units, each having a specified abbreviation or symbol (Table 9.1);
• derived units, obtained from combinations of base and supplementary units, which may also be given special symbols (Table 9.2);
• a set of prefixes to denote multiplication factors of 103, used for convenience to express multiples or fractions of units (Table 9.3).

Table 9. 1 The base and supplementary SI units

Table 9.3 Prefixes used in the SI

Table 9.2 Some important derived SI units