Base quantities




Base quantities are those quantities which are distinct in nature and in some cases have historically not been defined in terms of other quantities. Base quantities are those quantities on the basis of which other quantities can be expressed. The seven base quantities of the International System of Quantities (ISQ) and their corresponding SI units and dimensions are listed in the following table. Other conventions may have a different number of base units (e.g. the CGS and MKS systems of units).

International System of Quantities base quantities
Quantity SI unit Dimension
symbol
Name(s) (Common) symbol(s) Name Symbol
Length, width, height, depth, distance a, b, c, d, h, l, r, s, w, x, y, z metre m L
Time t, τ second s T
Mass m kilogram kg M
Absolute temperature T, θ kelvin K Θ
Amount of substance n mole mol N
Electric current i, I ampere A I
Luminous intensity Iv candela cd J
Plane angle α, β, γ, θ, φ, χ radian rad None
Solid angle ω, Ω steradian sr None

The last two angular units, plane angle and solid angle, are subsidiary units used in the SI, but are treated as dimensionless. The subsidiary units are used for convenience to differentiate between a truly dimensionless quantity (pure number) and an angle, which are different measurements.

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