## SI Base Units & Derived UnitsSI 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.
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.
For 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.
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 Note on degree Celsius. T, in terms of its difference from the reference temperature T_{0 }=
273.15 K, the ice point. This temperature difference is called a Celsius temperature, symbol t, and is
defined by the quantity equationt= T- T_{0}.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 byt/°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 difference t
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.
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