One kilogram is about 2.2046 pounds. That number is not a measurement or an approximation someone settled on: it falls out of a legal definition. Since 1959 the pound has been defined as exactly 0.45359237 kg, so every kilogram-to-pound conversion is just division by that number. This guide shows the formula, a chart you can print, three worked examples with real numbers, and the mental shortcuts worth remembering (plus how far off each one is).
The exact formula
The definition runs from pounds to kilograms, so start there:
- Pounds to kilograms: kg = lb × 0.45359237 (exact)
- Kilograms to pounds: lb = kg ÷ 0.45359237, which is the same as kg × 2.20462262… (the decimal never ends)
Because 1 ÷ 0.45359237 is an endless decimal, any "kg × 2.20462" figure you see is already rounded. For everyday use, five significant figures (2.2046) are more than enough: the error is about one part in 50,000, or roughly 2 grams on a 100 kg person.
Two related units come up constantly with pounds. An ounce is 1/16 of a pound, which works out to exactly 28.349523125 g. A stone, still used for body weight in the UK and Ireland, is 14 pounds, or exactly 6.35029318 kg. Both are fixed by the same definition, and the UK writes all three into law in Schedule 1 of the Weights and Measures Act 1985.
Kilograms to pounds chart
Every value below was computed from the exact definition and rounded to two decimals.
| Kilograms | Pounds | Kilograms | Pounds |
|---|---|---|---|
| 1 kg | 2.20 lb | 40 kg | 88.18 lb |
| 2 kg | 4.41 lb | 50 kg | 110.23 lb |
| 5 kg | 11.02 lb | 60 kg | 132.28 lb |
| 10 kg | 22.05 lb | 70 kg | 154.32 lb |
| 20 kg | 44.09 lb | 75 kg | 165.35 lb |
| 23 kg | 50.71 lb | 80 kg | 176.37 lb |
| 25 kg | 55.12 lb | 90 kg | 198.42 lb |
| 30 kg | 66.14 lb | 100 kg | 220.46 lb |
Going the other way is easier because the factor is exact:
| Pounds | Kilograms | Pounds | Kilograms |
|---|---|---|---|
| 1 lb | 0.454 kg | 50 lb | 22.680 kg |
| 5 lb | 2.268 kg | 100 lb | 45.359 kg |
| 10 lb | 4.536 kg | 150 lb | 68.039 kg |
| 25 lb | 11.340 kg | 180 lb | 81.647 kg |
| 45 lb | 20.412 kg | 200 lb | 90.718 kg |
Worked examples
1. A suitcase limit: 23 kg vs 50 lb
Baggage limits are often printed as "23 kg / 50 lb", as if the two were the same. They are close but not equal:
- 23 kg ÷ 0.45359237 = 50.71 lb
- 50 lb × 0.45359237 = 22.68 kg
So a bag that weighs 50.5 lb on a home scale is under a 23 kg limit but over a 50 lb one. If an airline states its limit in one unit, that unit is the one it will enforce, so weigh against it, and leave a small margin because scales differ.
2. Body weight in pounds, and in stones and pounds
Someone weighs 68 kg and wants the figure in US and UK styles:
- 68 ÷ 0.45359237 = 149.91 lb, so about 150 lb in US style.
- For UK style, divide the pounds by 14: 149.91 ÷ 14 = 10 stone with a remainder.
- The remainder is 149.91 − (10 × 14) = 9.91 lb, so the answer is 10 st 10 lb after rounding.
3. Gym plates
A standard Olympic barbell weighs 20 kg, which is 44.09 lb. American gyms usually stock 45 lb plates, which are 20.41 kg each. A bar loaded with one 45 lb plate per side is therefore 44.09 + 90 = 134.09 lb, or 60.82 kg, not the "135 lb" people usually quote, because the bar itself is under 45 lb. The difference is small, but it matters when you compare lifts logged in different units.
Before 1959 the US used the pound fixed by the 1893 Mendenhall Order, 0.4535924277 kg. The international pound adopted in 1959 is 0.45359237 kg, lighter by about 58 micrograms, or 0.13 parts per million. Nobody's bathroom scale could show the change, but calibration laboratories had to account for it.
Where the definition comes from
For most of the 19th and early 20th centuries, the United States, the United Kingdom and other English-speaking countries each kept slightly different pounds. The United States had defined its pound in terms of the kilogram since the Mendenhall Order of April 5, 1893. Britain kept its own physical imperial standard. On July 1, 1959, six countries (Australia, Canada, New Zealand, South Africa, the United Kingdom and the United States) agreed on a single international yard and pound. The US adopted it right away, while the UK and Australia made it legally effective on January 1, 1964.
The kilogram itself changed in 2019. Until then it was the mass of a metal cylinder kept near Paris, the International Prototype of the Kilogram. Since May 20, 2019 it has been defined by fixing the Planck constant at exactly 6.62607015 × 10−34 joule-seconds. Because the pound is defined as a fixed fraction of a kilogram, it moved to the new footing automatically, and the conversion factor did not change at all.
Mental math shortcuts and their errors
You rarely need eight decimals. These shortcuts work in your head; the error column shows how far each one is from the exact answer.
| Shortcut | Example: 80 kg | Error |
|---|---|---|
| Double it | 160 lb | 9.3% too low |
| Double it, add 10% (× 2.2) | 176 lb | 0.21% too low |
| × 2.2, then add 0.2% | 176.35 lb | 0.01% too low |
| Exact (÷ 0.45359237) | 176.37 lb | none |
For pounds to kilograms, "halve it and take off 10%" (× 0.45) is the usual trick. It comes out 0.79% low: 150 lb becomes 67.5 kg instead of 68.04 kg. That is fine for a recipe or a rough body weight, but not for a medication dose or a shipping quote.
Mass, weight and "pound-force"
Strictly, kilograms and pounds both measure mass, the amount of matter in something. Weight is a force, and in physics it is measured in newtons, or in the US engineering unit pound-force (lbf). On Earth the two are used interchangeably in daily life because a 1 kg mass always pulls on a scale with about the same force, so a bathroom scale can show either unit. The distinction matters in engineering: 1 lbf is defined as the force on 1 lb of mass under standard gravity, 9.80665 m/s², which is about 4.448 newtons. SamemaS keeps these apart: pounds are in the weight converter, and pound-force is in the force converter.
Using the SamemaS weight converter
The weight converter shows nine units at once: tonne, kilogram, gram, milligram, microgram, pound, ounce, stone and US ton. Type a value into any field and the others update, using the exact definitions above. Stone is shown as a decimal (10.71 st for 68 kg), not as stones plus pounds, so for the UK-style answer use the division step from example 2. For kitchen quantities measured by volume, such as cups of flour, you need a density as well as a unit factor; see our guide to US cups to grams.
The short version
- 1 lb = 0.45359237 kg exactly, and 1 kg ≈ 2.2046 lb.
- kg × 2.2 is within 0.21% of the exact value. Add 0.2% to get within 0.01%.
- 23 kg is 50.71 lb and 50 lb is 22.68 kg, so the two limits are not the same.
- 1 stone = 14 lb = 6.35029318 kg, and 1 oz = 28.349523125 g.
- For anything where a small error costs money or affects health, use the exact factor, not a shortcut.
If you also work with US customary lengths, our guide to metric and imperial systems around the world explains which countries use which.
Frequently asked questions
Sources
- International yard and pound — Wikipedia
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
- Weights and Measures Act 1985, Schedule 1 — legislation.gov.uk
- Mendenhall Order — Wikipedia
- 2019 revision of the SI — Wikipedia