SI derived unit
measurement using combination of base units
SI derived units are units of measurement derived from the
seven SI base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units, possibly scaled by an appropriate power of exponentiation (see: Buckingham π theorem). Some are dimensionless, as when the units cancel out in ratios of like quantities.
SI coherent derived units involve only a trivial proportionality factor, not requiring conversion factors.
The SI has special names for 22 of these coherent derived units (for example, hertz, the SI unit of measurement of frequency), but the rest merely reflect their derivation: for example, the square metre (m2), the SI derived unit of area; and the kilogram per cubic metre (kg/m3 or kg⋅m−3), the SI derived unit of density.
The names of SI coherent derived units, when written in full, are always in lowercase. However, the symbols for units named after persons are written with an uppercase initial letter. For example, the symbol for hertz is "Hz", while the symbol for metre is "m".
Special names
The International System of Units assigns special names to 22 derived units, which includes two dimensionless units, the radian (rad) and the steradian (sr).
By field of application
Kinematics
Mechanics
Chemistry
Electromagnetics
Photometry
Thermodynamics
Other units used with SI
Some other units such as the hour, litre, tonne, bar, and electronvolt are not SI units, but are widely used in conjunction with SI units.
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