The kilogram is the SI base unit of mass.It is defined as being equal to the mass of the international prototype of the kilogram which is kept in International Bureau of Weights and Measures.

Weight and mass are different. Mass is an intrinsic property of matter, whereas weight is a force that results from the action of gravity on matter. Since weight is a force, the SI unit of weight is simply the unit of force, namely the newton (N) – which can also be expressed in SI base units as kg·m/s² (kilograms times meters per second squared). Weight and mass are confused in daily life but it's not really important since gravity is almost same on earth and the gravitational force on an object (its weight) is directly proportional to its mass. It's said an object weighs 1 kilogram even though the kilogram is actually a unit of mass.

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