Quantities

A quantity is a property that can be measured or quantified. It is described by a magnitude, a numerical value, and a unit that specifies scale and dimension. Quantities are used across science and everyday life to express measurements such as length, mass, time, and temperature.

Quantities are categorized as base quantities and derived quantities obtained from base quantities via mathematical relationships. In the International System of Units (SI), base quantities include length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Derived quantities include speed, acceleration, force, energy, pressure, and many others. Quantities can be scalar, with only magnitude, or vector, with magnitude and direction.

Units provide a standard scale for measurement. Base units have well-defined standards, while derived units combine base units. Dimensional analysis uses units to check the consistency of equations. Measurements carry uncertainty, and reported values include an estimate of precision, often via significant figures or confidence intervals.

In practice, a quantity is expressed as value plus unit, and units may be converted using multiplication by conversion factors. Examples: 3.5 meters, 12.0 kilograms, or a speed of 20 meters per second. Understanding quantities and units enables consistent description, comparison, and computation across disciplines.