Quantity calculus is a powerful way of handling calculations involving physical quantities and their units. A physical quantity is considered to be the product of a numerical value (i.e. pure number) and a unit:

physical quantity = numerical value × unit

In this way, the units in which a physical quantity is measured are included in its specification, and they can be kept track of in calculations by applying the usual rules of algebra. For example, the wavelength of blue light, λ (the physical quantity) is about 450 nm, which may be written:

λ = 450 nm, or equivalently:  λ / nm = 450.

Now, since the units m and nm are related by nm = 10^-9 m,

λ = 4.5 × 10^-7 m, or equivalently:  λ / m = 4.5 × 10^-7

In equations, only pure numbers can be manipulated (if you doubt this, consider what could possibly be meant by raising a number to the power of a distance, or taking the natural logarithm of a temperature). Therefore, it is common in writing equations to specify both the physical quantity and its units. For example, to convert a temperature in in kelvin, T, to a temperature in degrees celsius, θ:

θ / °C = T / K - 273.15.

T is a physical quantity, say 298 K; T / K is a pure number, 298. So to apply the equation, subtracting the numbers 298 - 273.15 gives the number θ / °C = 24.85, and the physical quantity θ = 24.85 °C.

Quantity calculus is useful in tabulating the numerical values of physical quantities and in labelling the axes of graphs. For example:

The usual rules of algebra are followed to interpret tables like this. So, for example, the second entry in the second column, 10³ K / T = 5.000 implies that 1/T = 5 × 10^-3 K^-1. An equivalent way of writing this column heading would be (1/T) / 10^-3 K^-1.