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Conversion of units is the conversion of the unit of measurement in which a quantity is expressed, typically through a multiplicative conversion factor that changes the unit without changing the quantity. This is also often loosely taken to include replacement of a quantity with a corresponding quantity that describes the same physical property.
For measuring length, the U.S. customary system uses the inch, foot, yard, and mile, which are the only four customary length measurements in everyday use. From 1893, the foot was legally defined as exactly 1200 ⁄ 3937 m (approximately 0.304 8006 m). [13] Since July 1, 1959, the units of length have been defined on the basis of 1 yd = 0.9144 ...
The conversion between different SI units for one and the same physical quantity is always through a power of ten. This is why the SI (and metric systems more generally) are called decimal systems of measurement units. [10] The grouping formed by a prefix symbol attached to a unit symbol (e.g. ' km ', ' cm ') constitutes a new inseparable unit ...
Each variant of the metric system has a degree of coherence—the derived units are directly related to the base units without the need for intermediate conversion factors. [18] For example, in a coherent system the units of force , energy , and power are chosen so that the equations
The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units. Mesures usuelles (French for customary measures) were a system of measurement introduced as a compromise between the metric system and traditional measurements. It was used in France from 1812 to 1839.
Most of the units of measure have been adapted in one way or another since the Norman Conquest (1066). The units of linear measure have changed the least – the yard (which replaced the ell) and the chain were measures derived in England. The foot used by craftsmen supplanted the longer foot used in agriculture.
Conversions between units in the metric system are defined by their prefixes (for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article. Exceptions are made if the unit is commonly known by another name (for example, 1 micron = 10 −6 metre).
The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959. [2] [10] Common imperial units and U.S. customary units of length include: [11] thou or mil (1 ⁄ 1000 of an inch) inch (25.4 mm) foot (12 inches, 0.3048 m) yard (3 feet, 0.9144 m)