7 (International System of Units [SI] base units)

The International System of Units (SI) is founded on seven SI base units for seven base quantities assumed to be mutually independent, as given in Table 1. Here is the SI list from the NIST reference on Constants, Units and Uncertainty.

Table 1.  SI base units

SI base unit

Base quantity Name Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

For detailed information on the SI base units, see Definitions of the SI base units and their Historical context.

SI derived units

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted.

Table 2.  Examples of SI derived units

SI derived unit

Derived quantity Name Symbol
area square meter m2
volume cubic meter m3
speed, velocity meter per second m/s
acceleration meter per second squared m/s2
wave number reciprocal meter m-1
mass density kilogram per cubic meter kg/m3
specific volume cubic meter per kilogram m3/kg
current density ampere per square meter A/m2
magnetic field strength ampere per meter A/m
amount-of-substance concentration mole per cubic meter mol/m3
luminance candela per square meter cd/m2
mass fraction kilogram per kilogram, which may be represented by the number 1 kg/kg = 1

Table 3. SI derived units with special names and symbols
Table 4. Examples of SI derived units whose names and symbols include SI derived units with special names and symbols

For Table 3 and Table 4, click here.

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