The kilogram (also kilogramme) is the base unit of mass in the International System of Units (SI), the current metric system, having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo in everyday speech.
The kilogram was originally defined in 1795 as the mass of one litre of water. This definition was simple yet difficult to use in practice. However, even modern, superseding definitions of a kilogram are accurate to within 30 ppm of the mass of one litre of water.
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