A physical quantity has two essential attributes, namely magnitude and dimension. The first attribute is commonly expressed as a decimal number; for the second attribute one chooses a unit. The unit can be seen as the scale used for the specification of the magnitude of a physical quantity. For example, a certain length can be expressed as "5 dm" and as "50 cm".

Physical quantities may be expressed in base quantities. For example, speed is the distance travelled per unit of time. When we denote the dimension of length by the uppercase letter \(L\) and the dimension of time by the uppercase letter \(\mathrm{T}\), then the dimension of speed is denoted by \(\mathrm{L}/\mathrm{T}\) or \(\mathrm{LT}^{-1}\). Through the choice of units for length and time one gets a unit for speed. Velocity, defined as the rate of change of position with time, also has dimension \(\mathrm{LT}^{-1}\). Acceleration, defined as the rate of change of velocity with time, has dimension \(\mathrm{LT}^{-1}\times \mathrm{T}^{-1}=\mathrm{LT}^{-2}\) and a common choice of unit is \(\mathrm{m\,s}^{-2}\). Force has according to Newton's second law of motion the dimension of mass times acceleration \(\mathrm{M}\times \mathrm{LT}^{-2}=\mathrm{MLT}^{-2}\), with commonly used unit the newton \(\mathrm{N}=\mathrm{kg\,m\,s}^{-2}\).

The International System of Units, abbreviated SI system, is a metric system of standard units for measuring physical quantities such as length, mass, time, and temperature. The SI system uses seven, mutually independent base units in which all other units may be expressed.

\[\begin{array}{|l|l|l|l|l|} \hline
\mathit{Quantity} & \mathit{Symbol}& \mathit{Dimension} & \mathit{SI\mbox{-}unit} & \mathit{Symbol\;for\;SI\mbox{-}unit} \\ \hline
\mathrm{length} & l & \mathrm{L} &\mathrm{metre} & \mathrm{m} \\
\mathrm{mass} & m & \mathrm{M} & \mathrm{kilogram} & \mathrm{kg} \\
\mathrm{amount\;of\;substance} & n & \mathrm{N} & \mathrm{mole} & \mathrm{mol} \\
\mathrm{time} & t & \mathrm{T} &\mathrm{second} & \mathrm{s} \\
\mathrm{temperature} & T & \mathrm{\Theta} & \mathrm{kelvin} & \mathrm{K} \\
\mathrm{electric\;current} & I & \mathrm{I} & \mathrm{ampere} & \mathrm{A} \\
\mathrm{luminous\;intensity} & I_v & \mathrm{J} & \mathrm{candela} & \mathrm{cd} \\ \hline
\end{array}\]

The official unit of temperature is since 1967 equal to Kelvin and no longer as before degrees Kelvin (as in degrees Celsius); in older and popular literature you may still encounter the notation °K.