The seven SI base units and the interdependency of their definitions
The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:
- metre for length (US English: meter)
- kilogram for mass (note: not the gram)
- second for time
- ampere for electric current
- kelvin for temperature
- candela for luminous intensity
- mole for the amount of substance.
The SI base quantities form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. However, in a given realization of these units they may well be interdependent, i.e. defined in terms of each other.[1]
The names of all SI units are written in lowercase characters (e.g., the metre has the symbol m), except that the symbols of units named after persons are written with an initial capital letter (e.g., the ampere has the uppercase symbol A).
Many other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI.
SI Derived Units
The International System of Units (SI) specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre (m2), and of density is kilograms per cubic metre (kg/m3). The number of derived units is unlimited.
The names of SI units are always written in lowercase. The symbols of units named after persons, however, are always written with an uppercase initial letter (e.g., the symbol of hertz is Hz; but metre is m).
Examples of derived quantities and units
| Some SI derived units | ||||
|---|---|---|---|---|
| Name | Symbol | Quantity | Expression in terms of SI base units | |
| square metre | m2 | area | m2 | |
| cubic metre | m3 | volume | m3 | |
| metre per second | m/s | speed, velocity | m⋅s−1 | |
| cubic metre per second | m3/s | volumetric flow | m3⋅s−1 | |
| metre per second squared | m/s2 | acceleration | m⋅s−2 | |
| metre per second cubed | m/s3 | jerk, jolt | m⋅s−3 | |
| metre per quartic second | m/s4 | snap, jounce | m⋅s−4 | |
| radian per second | rad/s | angular velocity | s−1 | |
| newton second | N⋅s | momentum, impulse | m⋅kg⋅s−1 | |
| newton metre second | N⋅m⋅s | angular momentum | m2⋅kg⋅s−1 | |
| newton metre | N⋅m = J/rad | torque, moment of force | m2⋅kg⋅s−2 | |
| newton per second | N/s | yank | m⋅kg⋅s−3 | |
| reciprocal metre | m−1 | wavenumber | m−1 | |
| kilogram per square metre | kg/m2 | area density | m−2⋅kg | |
| kilogram per cubic metre | kg/m3 | density, mass density | m−3⋅kg | |
| cubic metre per kilogram | m3/kg | specific volume | m3⋅kg−1 | |
| mole per cubic metre | mol/m3 | amount of substance concentration | m−3⋅mol | |
| cubic metre per mole | m3/mol | molar volume | m3⋅mol−1 | |
| joule second | J⋅s | action | m2⋅kg⋅s−1 | |
| joule per kelvin | J/K | heat capacity, entropy | m2⋅kg⋅s−2⋅K−1 | |
| joule per kelvin mole | J/(K⋅mol) | molar heat capacity, molar entropy | m2⋅kg⋅s−2⋅K−1⋅mol−1 | |
| joule per kilogram kelvin | J/(K⋅kg) | specific heat capacity, specific entropy | m2⋅s−2⋅K−1 | |
| joule per mole | J/mol | molar energy | m2⋅kg⋅s−2⋅mol−1 | |
| joule per kilogram | J/kg | specific energy | m2⋅s−2 | |
| joule per cubic metre | J/m3 | energy density | m−1⋅kg⋅s−2 | |
| newton per metre | N/m = J/m2 | surface tension | kg⋅s−2 | |
| watt per square metre | W/m2 | heat flux density, irradiance | kg⋅s−3 | |
| watt per metre kelvin | W/(m⋅K) | thermal conductivity | m⋅kg⋅s−3⋅K−1 | |
| square metre per second | m2/s | kinematic viscosity, diffusion coefficient | m2⋅s−1 | |
| pascal second | Pa⋅s = N⋅s/m2 | dynamic viscosity | m−1⋅kg⋅s−1 | |
| coulomb per square metre | C/m2 | electric displacement field, polarization vector | m−2⋅s⋅A | |
| coulomb per cubic metre | C/m3 | electric charge density | m−3⋅s⋅A | |
| ampere per square metre | A/m2 | electric current density | A⋅m−2 | |
| siemens per metre | S/m | conductivity | m−3⋅kg−1⋅s3⋅A2 | |
| siemens square metre per mole | S⋅m2/mol | molar conductivity | kg-1⋅s3⋅mol−1⋅A2 | |
| farad per metre | F/m | permittivity | m−3⋅kg−1⋅s4⋅A2 | |
| henry per metre | H/m | permeability | m⋅kg⋅s−2⋅A−2 | |
| volt per metre | V/m | electric field strength | m⋅kg⋅s−3⋅A−1 | |
| ampere per metre | A/m | magnetic field strength | A⋅m−1 | |
| candela per square metre | cd/m2 | luminance | cd⋅m−2 | |
| lumen second | lm⋅s | luminous energy | cd⋅sr⋅s | |
| lux second | lx⋅s | luminous exposure | cd⋅sr⋅s/m−2 | |
| coulomb per kilogram | C/kg | exposure (X and gamma rays) | kg−1⋅s⋅A | |
| gray per second | Gy/s | absorbed dose rate | m2⋅s−3 | |
| ohm metre | Ω⋅m | resistivity | m3⋅kg⋅s−3⋅A−2 | |
