SI base units
The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent, as given in Table 1.
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.
ease of understanding and convenience, 22 SI derived units have been given
special names and symbols, as shown in Table 3.
For a graphical illustration of how the 22 derived units with special names
and symbols given in Table 3 are related to the seven SI base units, see
relationships among SI units.
Note on degree Celsius. The derived unit in Table 3 with the special name degree Celsius and special symbol °C deserves comment. Because of the way temperature scales used to be defined, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T0 = 273.15 K, the ice point. This temperature difference is called a Celsius temperature, symbol t, and is defined by the quantity equation
t= T- T0.
The unit of Celsius temperature is the degree Celsius, symbol °C. The numerical value of a Celsius temperature t expressed in degrees Celsius is given by
t/°C = T/K - 273.15.
It follows from the definition of t that the degree Celsius is equal
in magnitude to the kelvin, which in turn implies that the numerical
value of a given temperature difference or temperature interval whose
value is expressed in the unit degree Celsius (°C) is equal to the
numerical value of the same difference or interval when its value is
expressed in the unit kelvin (K). Thus, temperature differences or temperature
intervals may be expressed in either the degree Celsius or the kelvin
using the same numerical value. For example, the Celsius temperature
and the thermodynamic temperature difference T
between the melting point of gallium and the triple point of water may
be written as t
= 29.7546 °C = T
= 29.7546 K.
The special names and symbols of the 22 SI derived units with special names and symbols given in Table 3 may themselves be included in the names and symbols of other SI derived units, as shown in Table 4.
Continue to SI prefixes