To convert pounds to kilograms (lb to kg), multiply by 0.45359. 1 pound equals 0.45359 kg, about the same as a loaf of sandwich bread.
| pounds | kilograms |
|---|---|
| 1 pound | 0.45359 kg |
| 2 pounds | 0.90718 kg |
| 5 pounds | 2.2679619 kg |
| 10 pounds | 4.5359237 kg |
| 25 pounds | 11.339809 kg |
| 50 pounds | 22.679619 kg |
| 100 pounds | 45.359237 kg |
| 250 pounds | 113.39809 kg |
| 500 pounds | 226.79619 kg |
| 1000 pounds | 453.59237 kg |
Definition: Exactly 0.45359237 kilograms - 16 ounces.
History: The pound descends from the Roman "libra," which is also where the abbreviation "lb" comes from - it never matched the English word at all. It went through numerous competing regional definitions over the centuries until the "avoirdupois" pound was standardized, then finally locked to an exact metric value in 1959 by international agreement - meaning the pound has been officially defined in terms of the kilogram, not the other way around, for over 60 years.
Current use: Everyday weight in the US, including body weight, food packaging, and shipping. The UK largely moved to metric but still uses pounds, and stone, for body weight specifically.
Definition: The SI base unit of mass - 1,000 grams.
History: For 130 years, the kilogram was defined by a single physical object: a golf-ball-sized cylinder of platinum and iridium nicknamed "Le Grand K," locked in a vault outside Paris and taken out for cleaning only once every 40 years. Every kilogram on Earth was, technically, defined by how much that one specific object weighed - which became a real problem once scientists discovered it had lost about 50 micrograms of mass over its lifetime, for reasons nobody could fully explain. In 2019, the kilogram was redefined using Planck's constant instead, a fundamental property of physics that can't drift, get scratched, or need cleaning.
Current use: The standard unit of mass in nearly every country on Earth, used in science, medicine, and everyday commerce.
A pound is defined as exactly 0.45359237 kilograms, agreed in 1959. The conversion is exact; the decimal is just long.
No simple round multiplier is close enough to be useful, so this is one to look up rather than estimate. The exact factor is 0.453592.
Going the other way, one kilogram is 2.205 lb, which is usually the easier direction to work in. Because the forward factor is small, watch the decimal place rather than the digits: a factor-of-ten slip is the common error, not a rounding one.
To anchor the scale: 5 pounds is about 2.27 kg.