What is "International System of Units (SI)"?

International System of Units, or Système International d'Unités (SI), is a standard system of units maintained by the International Bureau of Weights and Measures.

SI can be summarized as:

Seven Base Units - Seven base units are specified in SI. One for each base quantity (quantity type): m (meter for length), kg (kilogram for mass), s (second for time), A (ampere for electric current), K (kelvin for thermodynamic temperature), mol (mole for amount of substance) and cd (candela for luminous intensity).

Coherent Units - For each derived quantity (quantity type) one coherent unit is defined in the form of products of powers of the base units. For example, m⋅s⁻¹ is the coherent unit for speed. kg⋅m⋅s⁻² is the coherent unit for force, named as N (newton for force).

Prefixed Units - SI specifies 24 prefixes which, when added to the name and symbol of a coherent unit, produce 24 prefixed units for the same quantity. But there are some exceptions like kg for mass, which is already in a prefix form. Those prefixed units are always decimal (i.e. power-of-ten) multiples of the coherent unit. For example, mm (millimeter) = 10^-3 m is a prefixed unit for length. kN (kilonewton) = 10³ N is a prefixed unit for force.

Derived Units - There are several commonly used units that are included in SI as derived units. For example, °C (degree Celsius) is a derived unit for thermodynamic temperature.

ISQ Units - There are many other commonly used units that are not included in SI. But they are included in the ISQ (International System of Quantities) standard. For example, h (hour) is non-SI unit for time.

⇒ SI Defining Constants

⇐ International System of Quantities (ISQ)

⇑ ISO/IEC 80000 - Quantities and units

⇑⇑ MathML (Mathematical Markup Language)