International System of Quantities is that system of quantities based on the seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

The definition of the units corresponding to the base quantities is extremely important since they provide the foundation for the entire system of units . The International System of Units, the SI, is recognised by many countries (at the time of writing there are 64 members states of the Metre Convention) and is adopted as the only legal system of units within the European Union . The base quantities and their corresponding base units are shown in Table 1. In 2018 it was agreed that four of these base units would be redefined – the kilogram, the mole, the ampere and the kelvin. Following the adoption of the revised definitions in May 2019, all the base units are defined in terms of fundamental constants which are unchanged with respect to time and location. By definition, these fundamental constants have no uncertainty. The kilogram is defined in terms of the Planck constant h = 6.626 070 15 × 10-34 J s and the mole as the specific number of elementary entities (atoms, molecules, ions, etc.) given by the Avogadro constant NA = 6.022 140 76 × 1023 mol-1. These changes do not affect the way routine measurements are made and metrological traceability is obtained exactly as before. It is only the definition of the units that has changed. Many other quantities within the SI are expressed as relations between those shown in Table 1 and are called derived quantities (VIM 1.5). The definitions of the derived units in terms of the base units follow from the equations defining the derived quantities in terms of the base quantities. For example, the derived quantity mass density (ρ) is:

ρ = m/V

where m is mass (expressed in kg) and V is volume

(expressed in m3).

The measurement unit (derived unit) is obtained by applying the same formula to the units, i.e.

kg/m3 which is usually written as kg m-3 or kg/m3 A common mistake is to confuse quantities and their measurement units. It should be kept in mind that whereas a quantity is a measurable property of a phenomenon, body or substance (e.g. mass), a measurement unit (e.g. the kilogram) is chosen by convention as the reference to which measurements of that property refer. For example, it is not consistent with recommended SI usage to describe a measurand as ‘number of moles’. In this instance, the quantity is ‘amount of substance’, and the unit is the mole.

ref of International System of Quantities: Terminology in Analytica Measurement Introduction to VIM 3