Kilogram

The kilogram is the SI base unit of mass. One kilogram equals about 2.20462 pounds.

Until 2019 the kilogram was defined by a physical object — the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept in a vault in Sèvres, France since 1889. Every other mass measurement on Earth was, ultimately, traceable to that artifact.

The problem: the IPK and its official copies were drifting in mass over time, on the order of 50 micrograms per century. For most practical use this was invisible; for the most precise scientific work it was a hard ceiling on accuracy.

In May 2019 the kilogram was redefined using a fixed value of the Planck constant (6.62607015 × 10⁻³⁴ joule-seconds, exact by definition). The new definition is based on a fundamental physical constant rather than an artifact. Any laboratory equipped with a Kibble balance can now realise the kilogram independently — no central reference object needed.

Practical conversions: 1 kg = 1000 g; 1 lb = 0.45359237 kg exactly; 1 stone (UK) = 6.35029 kg; 1 metric ton = 1000 kg; 1 US ton = 907.185 kg; 1 UK long ton = 1016.05 kg.