2.01 SI Units

As previously stated the actual property we are measuring (e.g. length, temperature etc) is called a quantity and in order to measure quantities we need to use clearly defined units such as metres, kilograms etc.

The units we use for measurement in science are called SI units. These units are specified by the International Bureau of Weights and Measures who are based at Sevres in France. The SI units of measurements for each measured quantity are listed in the following sections.

Note. You should not try to learn all of these units from the outset. The relevant units will be introduced as you progress through your studies.


Your objectives should be to

The seven base SI units

Base unit Symbol Physical quantity
metre m Length
second s Time
kilogram kg Mass
ampere A Electric current
Kelvin K Temperature
candela cd Luminous intensity
mole mol Amount of substance

The size of each of these base units is set by the experimental measurement of a physical quantity under specific conditions at which it always has the same constant value. In addition to the quantities measured directly by each base unit, all other physical quantities can be measured using different combinations of these base units.