The kilogram (abbreviation, kg) is the Standard International (SI) System of Units unit of mass. It is defined as the mass of a particular international prototype made of platinum-iridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10-3 cubic meter) of pure water.
At the Earth's surface, a mass of 1 kg weighs approximately 2.20 pounds (lb). Conversely, an object that weighs 1 lb at the Earth's surface has a mass of approximately 0.454 kg. There is a qualitative as well as a quantitative difference between kilograms and pounds. Kilograms denote mass, but pounds denote weight, the force a mass exerts against a barrier in the presence of an acceleration field acting perpendicular to the barrier. On the surface of Mars, where the gravitational acceleration is 37 percent of that on the surface of the Earth, a mass of 1 kg weighs 0.814 lb. In orbit, or in a vehicle coasting through space, the net acceleration is zero and a mass of 1 kg has no weight. In other words, it is weightless.
Mass is often specified in smaller units than the kilogram, by changing the power-of-10 prefix multiplier. A mass of one gram (1 g) is 10-3 kg. A mass of one milligram (1 mg) is 10-6 kg, or 10-3 g. A mass of one microgram (1 µg) is 10-9 kg, or 10-6 g. A mass of one nanogram (1 ng) is 10-12 kg, or 10-9 g.