For any measurement of a quantity such as distance, time, mass, force, etc. it is important to specify the units of the measurement. A distance of 3 is meaningless unless we indicate whether we mean 3 metres, 3 kilometres or 3 Megaparsecs.

The most widely used set of units is known variously as the Systéme Internationale, International System of Units or SI Units. These units are a modernised version of the metric system, and make use of 7 base units:

Measurement Name Symbol Length Metre m Mass Kilogram kg Time Second s Current Ampere A Thermodynamic Temperature Kelvin K Quantity of substance Mole mol Luminous Intensity Candela cd The SI units are complemented by two angular units:

Measurement Name Symbol Plane angle radian rad Solid angle steradian sr It should be noted that two of the SI units are somewhat redundant. The mole is a measure of the number of atoms or molecules, so it is not really a unit. Candelas can be specified in terms of m, kg and s (1 Cd = 1/683 W sr

^{-1}= 1/683 kg m^{2}s^{-3}sr^{-1}at 5.40 × 10^{14}Hertz).Commonly derived units include:

Measurement Name Symbol Derived Unit Force Newton N m kg s ^{-2}Pressure Pascal Pa N m ^{-2}Energy Joule J N m Power Watt W J s ^{-1}Electric charge Coloumb C A s