One kilogram is 2.2046226218 pounds. Like most metric-to-imperial conversions this one is exact in the other direction: the pound is defined as exactly 0.45359237 kilograms, and the figure above is simply one divided by that.
About the units
The kilogram is the SI base unit of mass, and it is the unit with the most interesting recent history. Until 2019 it was the last SI unit still defined by a physical artefact — a platinum-iridium cylinder kept outside Paris since 1889, which was slowly and inexplicably losing mass relative to its own official copies. In May 2019 the definition was replaced: the Planck constant was fixed at exactly 6.62607015 × 10⁻³⁴ joule-seconds, and the kilogram now follows from that. Nothing changed for anyone weighing groceries, but the unit stopped depending on an object that could be dropped. The pound, more precisely the international avoirdupois pound, has a commercial rather than scientific ancestry — it comes from medieval wool trading — and like the foot it was only pinned down internationally in 1959.
The exact factor
Since 1 lb = 0.45359237 kg exactly, converting pounds to kilograms terminates cleanly. The reverse, 1 ÷ 0.45359237 = 2.20462262184877..., does not. The everyday shortcut most people use is "double it and add ten per cent", which gives 2.2 and is accurate to about 0.2 per cent — close enough for body weight, wrong enough to matter for freight. Note also that the pound is a unit of mass here, not of force. The pound-force, which appears in engineering contexts, is a different quantity entirely and lives in the force category on this site; conflating the two is one of the classic sources of silent error in mechanical calculations.
Where you meet this conversion
Body weight is the most common encounter — American medical records, gym equipment and airline baggage limits all use pounds, while the rest of the world uses kilograms. Shipping and freight documents routinely carry both, and mismatches between them are a recurring source of customs delays. In cooking, American recipes give meat and produce in pounds while European ones use kilograms and grams. Bear in mind the British habit of expressing body weight in stones and pounds, where a stone is 14 pounds: "eleven stone four" is 158 pounds, about 71.7 kilograms.