Convert pounds to kilograms

One pound equals exactly 0.45359237 kilograms. Like the inch, it is exact by definition — the same 1959 agreement fixed both, and the figure applies to the avoirdupois pound used for everyday weighing.

Because it is 50 pounds rounded. A 50 lb allowance is 22.68 kg, and carriers in metric countries round up to a clean 23. The oddly specific number on your airline's website is an imperial rule wearing metric clothes.

It stopped being a physical object. Until May 2019, the kilogram was the last SI base unit defined by an artifact: a platinum-iridium cylinder cast in 1879, kept under three glass bells in a vault at Sèvres, near Paris, and known as Le Grand K. Over the twentieth century it drifted about 50 micrograms — roughly a grain of sand — against the official copies held by other countries.

A standard that changes is not a standard, so the General Conference on Weights and Measures redefined the kilogram in terms of the Planck constant, measured with an instrument called a Kibble balance. The definition took effect on World Metrology Day, 20 May 2019. Your bathroom scale did not notice.

Halving. The mental shortcut turns 150 lb into 75 kg, but the real answer is 68 kg — the shortcut overshoots by about 10 percent. Fine for guessing, not fine for a medication dose or a freight quote. Doses themselves are often in micrograms, where a similar thousandfold error is possible — see micrograms to grams.

The other trap is the troy pound, still used for precious metals. It holds 12 troy ounces and weighs about 373 g, not 454 g. A pound of gold and a pound of flour are different weights.

The national laboratories that signed the 1959 international yard and pound agreement. The US National Bureau of Standards (now NIST), in an announcement effective 1 July 1959, fixed the avoirdupois pound at exactly 0.45359237 kg; the United Kingdom defines the same pound in its Weights and Measures Act 1985, and NIST Special Publication 811, the US guide to the SI lists it as exact.