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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).
historical definitions of the units and their derivatives used in old measurements; e.g., international foot vs. US survey foot. For some purposes, conversions from one system of units to another are needed to be exact, without increasing or decreasing the precision of the expressed quantity.
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
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] [5] Common imperial units and U.S. customary units of length include: [6] thou or mil (1 ⁄ 1000 of an inch) inch (25.4 mm) foot (12 inches, 0.3048 m) yard (3 feet, 0.9144 m)
The International System of Units is the modern metric system. It is based on the metre–kilogram–second–ampere (MKSA) system of units from early in the 20th century. [ 20 ] It also includes numerous coherent derived units for common quantities like power (watt) and irradience (lumen).
The solar flux unit is a unit of spectral irradiance equal to 10 −22 W⋅m −2 ⋅Hz −1 (100 yW⋅m −2 ⋅Hz −1). The nox (nx) is a unit of illuminance equal to 1 millilux (1 mlx). The nit (nt) is a unit of luminance equal to one candela per metre squared (1 cd⋅m −2). The lambert (L) is a unit of luminance equal to 10 4 /π cd⋅m ...
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The International System of Units, internationally known by the abbreviation SI (from French Système international d'unités), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official status in nearly every country in the world, employed in science ...