Quantity, property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference (VIM1.1)

Quantity is a crucial concept in metrology, which applies across all disciplines involved with measurement, and is therefore the first term defined in VIM 3 [1]. The definition identifies a quantity as any property which has size (magnitude). The magnitude is usually evaluated through measurement but can also be fixed by definition, for example the speed of light.

There are many kinds of quantity including mass, volume, velocity (speed), electric current and flow rate. In everyday life, we are interested in specific examples of such quantities (formerly referred to as ‘particular quantities’) [10], e.g. the volume of gasoline dispensed into a vehicle, the speed at which my car was travelling when the police stopped me, or the number concentration of red cells in the blood sample taken yesterday from Mr. Smith.

The specification of the (particular) quantity we intend to measure (also called the measurand) is the first part of any measurement.

## Quantities of the same kind

VIM defines the concept ‘kind of quantity’ (VIM 1.2) or ‘kind’. Quantities of the same kind will have the same unit but two quantity values having the same unit do not have to be of the same kind. The unit of mass density and of mass concentration is kg m-3 but these are not quantities of the same kind. The measurement unit of both frequency and activity of radionuclides is s-1 but they are not quantities of the same kind. In this example the unit in each case is given a special name, namely hertz (Hz) and becquerel (Bq), respectively.

ref: Terminology in Analytica Measurement Introduction to VIM 3