Weight Converter (kg to lbs)
Convert weight between kilograms and pounds using the internationally defined conversion factor.
What this tool does
This converter turns a weight between kilograms and pounds using the exact factor 1 lb = 0.45359237 kg, fixed by the 1959 international yard and pound agreement, which makes 1 kg = 2.20462262185 lb. Enter a weight in either unit using the toggle above the field, and it returns the equivalent along with grams, stone, and the mixed forms these weights are actually spoken in: stone and pounds, and pounds and ounces. The factor is a definition rather than a measurement, so the conversion carries no uncertainty at any scale.
Formula Used
Disclaimer
This calculator is for educational and informational purposes only. It does not provide medical, nutritional, or training advice. Results are mathematical estimates and may not reflect individual circumstances. Consult a qualified coach, registered dietitian, medical professional, or physiotherapist for personal guidance.
How the Weight Converter works
Enter a weight and use the unit toggle above the field to say whether it is kilograms or pounds. The tool returns the equivalent in the other unit plus grams, stone, and two mixed forms: stone and pounds, and pounds and ounces. Those last two exist because they are how the numbers are said aloud. A British bodyweight is quoted as twelve stone eight, not 12.60 stone, and a US weight as 176 pounds 6 ounces rather than 176.37 pounds.
The formula
1 lb = 0.45359237 kg exactly, so lb = kg ÷ 0.45359237 and kg = lb × 0.45359237. The inverse works out to 1 kg = 2.20462262185 lb. Working the default: 80 ÷ 0.45359237 = 176.37 lb, which is 12.60 stone, or 12 st 8.4 lb, or 176 lb 5.9 oz, or 80,000 g.
Why the factor is exact
It is a definition, not a measurement. In 1959 the national standards bodies of the English-speaking countries agreed to define the international avoirdupois pound as exactly 0.45359237 kilograms, and the yard as exactly 0.9144 metres. The notice appears in the United States Federal Register of 1 July 1959 at 24 FR 5348 (National Bureau of Standards, 1959). Because the pound is defined against the kilogram, and the kilogram is an SI base unit (BIPM, SI Brochure), the conversion has no uncertainty of its own and behaves identically at 0.5 kg and at 500 kg. The United Kingdom carries the same definition in Schedule 1 of the Weights and Measures Act 1985 (UK legislation).
Why not just use 2.2
Because the error compounds with size. At 80 kg, multiplying by 2.2 gives 176.00 lb against a true 176.37, a shortfall of 0.37 lb. At 100 kg it is 220.00 against 220.46, and the gap keeps widening at roughly 0.21% of whatever you started with. For a rough mental estimate 2.2 is fine; for a plate load, a shipping weight, or anything recorded and compared later, the extra digits cost nothing and remove the drift.
Stone, and where it is still used
One stone is 14 pounds, a unit that survives mainly for bodyweight in the United Kingdom and Ireland. Its decimal form is nearly useless in conversation, which is why the tool also gives the stone-and-pounds pair: 80 kg is 12.60 st in decimal and 12 st 8.4 lb as anyone would actually say it. Note that stone is a pounds-derived unit, so it inherits the same exact definition rather than needing one of its own.
What this tool does not do
It converts mass between avoirdupois units and the metric system. It does not handle troy weight, used for precious metals, where the ounce is 31.1035 g against the avoirdupois 28.3495 g, nor apothecary units or metric tonnes. It says nothing about body composition, health, training readiness, or whether any particular weight is appropriate for anyone. The output is a mathematical transformation and not a recommendation.
Disclaimer
This tool is provided for educational and informational purposes only. It is not medical, nutritional, or training advice. The calculator applies a standard unit conversion formula and does not account for individual circumstances, health conditions, training history, or personal goals. Consult qualified professionals for personalised guidance. No calculator output should be interpreted as instruction or endorsement of any specific action.
Questions
- Why use 2.20462 rather than 2.2?
- Because 2.2 drifts, and it drifts proportionally. At 80 kg it gives 176.00 lb against a true 176.37, and at 100 kg it gives 220.00 against 220.46, an error of about 0.21% at any size. That is invisible for a rough estimate and not invisible on a barbell or a scale reading you intend to compare against later. The precise value follows from the pound being defined as exactly 0.45359237 kg.
- What are the stone-and-pounds and pounds-and-ounces rows for?
- They are the forms these weights are spoken in. Nobody says 12.60 stone; they say twelve stone eight, which is what 80 kg comes to. The same goes for 176 lb 5.9 oz rather than 176.37 lb. The decimal versions stay because they are what you type into a spreadsheet, but the mixed forms are what you say out loud.
- Does the conversion factor change at different weights?
- No. It is a fixed definition rather than an empirical measurement, so it applies identically from a gram to a tonne. The pound was defined as exactly 0.45359237 kg by international agreement in 1959, which means the conversion introduces no error of its own at any magnitude; whatever inaccuracy you see comes from the scale, not the arithmetic.
- Can it convert troy ounces?
- No. This is an avoirdupois converter, covering pounds, ounces, stone, kilograms and grams. Troy weight, used for precious metals, is a separate system: a troy ounce is 31.1035 g against the avoirdupois ounce's 28.3495 g, so mixing them up misprices gold by about 10%. Apothecary units and metric tonnes are outside the tool as well.
- How do I convert in the other direction?
- Use the unit toggle above the input field to switch it to pounds, then type the pound figure. The tool converts it and shows the kilogram equivalent alongside every other form. Entering 180 with the toggle set to pounds returns 81.65 kg, which is 180.00 lb back again.
- Where is stone actually used?
- Almost exclusively for human bodyweight in the United Kingdom and Ireland, and even there mostly in speech rather than in medical records, which use kilograms. It is not used in the United States, where the same weight is given in pounds alone, nor anywhere metric. That regional split is the reason the tool shows both alongside the kilogram figure rather than picking one.
Sources & Methodology
Applies the exact international factor 1 lb = 0.45359237 kg, fixed by the 1959 international yard and pound agreement and announced in the US Federal Register of 1 July 1959 at 24 FR 5348, which makes 1 kg = 2.20462262185 lb. Stone is pounds ÷ 14. Mixed forms are derived from the decimal ones: stone and pounds, and pounds and ounces at 16 oz to the pound. Because the pound is defined against the kilogram, the conversion is exact at every magnitude.
- › National Bureau of Standards. Refinement of Values for the Yard and the Pound. Federal Register, 1 July 1959; 24 FR 5348.
- › Bureau International des Poids et Mesures. The International System of Units (SI), 9th edition, 2019.
- › Weights and Measures Act 1985, Schedule 1: definitions of units. UK legislation.
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