unitsandquantities

Units and quantities are the framework used to measure and describe physical properties. A quantity is a property that can be measured, defined by a numeric magnitude and a unit. The magnitude is a number, while the unit provides the reference for that magnitude, enabling consistent communication of measurements.

The International System of Units (SI) defines seven base units: metre for length, kilogram for mass, second for time, ampere for electric current, kelvin for thermodynamic temperature, mole for amount of substance, and candela for luminous intensity. Most practical quantities are described by derived units, formed from base units. Examples include velocity (metre per second), area (square metre), force (newton, kg·m/s^2), and energy (joule, kg·m^2/s^2).

Prefixes such as kilo-, milli-, and micro- modify units to express large or small quantities without changing the unit type. Unit symbols (m, s, kg, A, K, mol, cd) standardize representation across contexts. Some quantities are dimensionless, such as ratios or constants, and carry no units.

Dimensional analysis uses units to check the consistency of equations and computations. Unit conversion converts measurements from one unit to another using conversion factors, ensuring results remain dimensionally consistent. Measurements carry uncertainty, which is reported alongside the value; units themselves do not convey precision but are essential for interpretation. Metrology, the science of measurement, governs units, standards, and their reliable application across science, industry, and daily life.