Definition of si unit system

SI is the International System of Units, a coherent system used to express measurements. Its seven base units include the second, meter, kilogram, ampere, kelvin, mole, and candela. Other units are derived through defined relationships.

Meaning and context

The units correspond to different quantities: time, length, mass, electric current, thermodynamic temperature, amount of substance, and luminous intensity. A numerical value needs an appropriate unit to communicate what has been measured.

Examples and usage

Speed can be expressed in meters per second, combining a length unit and a time unit. Prefixes such as kilo and milli represent decimal factors. A kilometer is one thousand meters, while a millimeter is one thousandth of a meter.

What to check

Unit names, symbols, capitalization, and prefixes have conventions. Changing case can change a symbol’s meaning. A conversion should preserve the physical quantity while changing the unit used to express it; it should not turn a length into an area or a mass into a volume.

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