A physical quantity (or simply quantity) is a property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a value, which is the algebraic multiplication of a numerical value and a unit of measurement. For example, the physical quantity mass, symbol m, can be quantified as m=n kg, where n is the numerical value and kg is the unit symbol (for kilogram). Vector quantities have, besides numerical value and unit, direction or orientation in space.

The notion of dimension of a physical quantity was introduced by Joseph Fourier in 1822. By convention, physical quantities are organized in a dimensional system built upon base quantities, each of which is regarded as having its own dimension. The dimension of a quantity Z is denoted dim Z or dim(Z).

Dimensional homogeneity is not necessarily sufficient for quantities to be comparable; for example, both kinematic viscosity and thermal diffusivity have dimension of square length per time (in units of m²/s). Quantities of the same kind share extra commonalities beyond their dimension and units allowing their comparison; for example, not all dimensionless quantities are of the same kind ().

There is often a choice of unit, though SI units are usually used in scientific contexts due to their ease of use, international familiarity and prescription. For example, a quantity of mass might be represented by the symbol m, and could be expressed in the units kilograms (kg), pounds (lb), or daltons (Da). The unit of a quantity Z is denoted [Z].

Following ISO 80000-1, any value or magnitude of a physical quantity is expressed as a comparison to a unit of that quantity. The value of a physical quantity Z is expressed as the product of a numerical value {Z} (a pure number) and a unit [Z]:

For example, let ${\displaystyle Z}$ be "2 metres"; then, ${\displaystyle \{Z\}=2}$ is the numerical value and ${\displaystyle [Z]=\mathrm {metre} }$ is the unit. Conversely, the numerical value expressed in an arbitrary unit can be obtained as:

The multiplication sign is usually left out, just as it is left out between variables in the scientific notation of formulas. The convention used to express quantities is referred to as quantity calculus. In formulas, the unit [Z] can be treated as if it were a specific magnitude of a kind of physical dimension: see Dimensional analysis for more on this treatment.

International recommendations for the use of symbols for quantities are set out in ISO/IEC 80000, the IUPAP red book and the IUPAC green book. For example, the recommended symbol for the physical quantity "mass" is m, and the recommended symbol for the quantity "electric charge" is Q.