Kilogram (kg)
Definition
The kilogram (symbol kg) is the base unit of mass in the International System of Units (SI). It measures the quantity of matter in an object and is defined by fixing the numerical value of the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds.
History
The kilogram was originally defined in 1795 as the mass of one liter of water at the temperature of melting ice, then redefined in 1799 against a platinum cylinder known as the Kilogramme des Archives. In 1889 the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder, became the legal artifact at the BIPM near Paris.
By the mid-twentieth century, comparisons showed the IPK and its copies drifting by tens of micrograms relative to each other. After more than two decades of preparation, the 2019 SI revision retired the physical artifact and defined the kilogram instead by fixing the Planck constant. The change took effect on 20 May 2019, World Metrology Day.
Standard reference
Defined by the BIPM SI Brochure (9th edition, 2019) by fixing the Planck constant h at 6.62607015 × 10⁻³⁴ J·s exactly. NIST Special Publication 811 governs U.S. usage.
Common conversions
| 1 kg | = 1,000 g |
|---|---|
| = 2.20462 lb | |
| = 35.274 oz | |
| = 0.157473 st |