Units

Units are standard quantities used to express physical quantities. They provide a magnitude and a dimension, allowing precise measurement, communication, and calculation. A quantity is described by a number and its unit, for example 5 meters or 12 seconds.

The International System of Units (SI) is the global standard for science and most everyday measurements. SI defines seven base units: the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity. Most measurements are expressed as derived units formed from these bases, such as the newton for force (kg·m/s^2), the joule for energy (kg·m^2/s^2), and the watt for power (kg·m^2/s^3). The SI system is coherent, meaning derived units are obtained directly from base units without extra conversion factors.

Prefixes are used to express multiples and submultiples of units, enabling convenient handling of very large or very small values. Common SI prefixes include kilo- (10^3), centi- (10^-2), milli- (10^-3), micro- (10^-6), and nano- (10^-9).

Redefinitions and non-SI units are part of practical use. In 2019 the kilogram was redefined in terms of the Planck constant. Some widely used units, such as inch, pound, or liter, lie outside SI and follow regional conventions.

Conversions and dimensional analysis are central to working with units. Conversion factors relate different units; dimensional analysis checks that equations are consistent and quantities balance correctly.