Physical quantity

Base and derived quantities

Physical quantities are commonly classified into base (fundamental) quantities and derived quantities. Base quantities are chosen as a minimal set from which other quantities are constructed by multiplication, division or exponentiation. The seven internationally accepted base quantities of the SI system include:

• mass
• length
• time
• temperature
• electric current
• luminous intensity (light intensity)
• amount of substance

Derived quantities are combinations of base quantities. For example, velocity is length divided by time, acceleration is change of velocity per time, and force can be expressed in terms of mass, length and time. Other common derived quantities include density, pressure and energy. Some derived quantities are dimensionless (pure numbers) and appear as ratios or angles in equations.

Units, dimensions and measurement

Every physical quantity is reported as a number together with a unit. Units (metre, second, kilogram, kelvin, ampere, mole, candela in SI) are standard references for measurement. The dimensional formula of a quantity records how it depends on base quantities; this helps check equations and convert between unit systems. Accurate measurement requires clear operational definitions, calibrated instruments, and an estimate of uncertainty. The process of determining or comparing quantities underlies experimental physics and engineering and is the basis of traceability to international standards.

Quantities may be scalar (magnitude only) or vectorial (magnitude and direction). Examples: temperature is a scalar, while velocity is a vector. Some quantities, like power or concentration, are extensive or intensive depending on whether they scale with system size. Understanding these distinctions guides correct use in formulas and experiments.

Historically, the formalization of physical quantities and units evolved as science and technology demanded consistent communication and reproducible results. International agreement on base quantities and the SI system has simplified conversion and reduced ambiguity. Today, the language of physical quantities — units, dimensions, and well-defined measurement procedures — is essential across research, industry, and education where quantitative description is required.

For further study, typical entries and definitions are available for many individual quantities and units; examples include mass, length, time, temperature, electric current, luminous intensity, amount of substance, and commonly used derived quantities like force, velocity, density and acceleration. Additional resources and standards can be explored through technical and metrology references.

Note: measurement practice and terminology may vary slightly between disciplines; precise definitions and conventions are supplied by standards organizations and technical literature.