Kilogram

:For other uses of 'kg' see kg (disambiguation)

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The kilogram or kilogramme, (symbol: kg) is the SI base unit of mass. It is defined as being equal to the mass of the international prototype of the kilogram.

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It is the only SI base unit that employs a prefix, and the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property.

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The kilogram was originally defined as the mass of one litre of pure water at a temperature of 3.98 degrees Celsius and standard atmospheric pressure.

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This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.

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