The weight conversion problem
Weight is one of those measurements where the unit you grew up with feels obvious and every other unit feels foreign. An American knows what 150 pounds feels like and has no intuition for 68 kilograms. A British person thinks of body weight in stone — 11 stone 11 — and finds pounds-only measurements oddly granular. A continental European has used kilograms their whole life and can't imagine why anyone would split a single quantity across two different units.
The Weight Converter handles all of them: kilograms, grams, pounds, ounces, stone, US tons, and metric tonnes. Type a value in any unit, pick the unit you want to convert to, and the answer updates. Useful for shipping labels, recipes, gym progress, and the moment you book an international flight and the baggage allowance is in kilograms but the scale at home is in pounds.
Like length conversions, weight conversions are exact rather than approximate. Since 1959, one pound has been defined as exactly 0.45359237 kilograms — the international avoirdupois pound. Every other imperial weight unit derives from that.
How to use the Weight Converter
- Type your weight into the input box.
- Pick the unit you're starting from — kilograms, pounds, stone, and so on.
- Pick the unit you want the answer in.
- Read the converted value below.
The result updates as you type — no submit button. Calculations run in your browser, so your weight doesn't get sent anywhere. The tool covers metric (mg, g, kg, tonne) and imperial / US customary (oz, lb, stone, US ton).
The exact conversions that matter
1 pound = 0.45359237 kg exactly (so 1 kg = 2.20462262 lb)
1 ounce = 28.3495 g (more precisely, 28.349523125 g)
1 stone = 14 pounds = 6.35029 kg
1 metric tonne = 1000 kg = 2204.62 lb
1 US ton = 2000 lb = 907.185 kg
The pound-to-kilogram conversion is exact because the international yard and pound agreement of 1959 defined it that way. The constant 0.45359237 isn't a measurement; it's a definition. Before 1959, the US and UK had slightly different "pounds," and the agreement merged them. The metric system serves as the reference, and imperial units are now defined in terms of kilograms.
Worked example: 165 lb in metric and stone
A common case: you weigh 165 pounds on an American scale and you need to fill in a form that asks for kilograms (medical record, international flight, gym membership in Europe). Or you're chatting with a British friend who asks your weight in stone.
Kilograms first. Multiply by the exact conversion factor:
- 165 lb × 0.45359237 kg/lb = 74.84 kg
For the rule-of-thumb shortcut: divide pounds by 2.2. That gives 165 / 2.2 = 75.0 kg, which is 0.16 kg high. Close enough for conversation, not close enough for a prescription.
Stone next. One stone is 14 pounds, so divide pounds by 14:
- 165 / 14 = 11.79 stone, which is 11 stone and 0.79 × 14 = 11 stone 11 lb
So 165 lb is 74.84 kg, or 11 stone 11 lb. Three numbers, one person, different countries.
Common weight conversion table
| From | To | Multiply by |
|---|---|---|
| pounds | kilograms | 0.453592 |
| kilograms | pounds | 2.20462 |
| ounces | grams | 28.3495 |
| grams | ounces | 0.0352740 |
| pounds | ounces | 16 |
| stone | pounds | 14 |
| stone | kilograms | 6.35029 |
| US tons | kilograms | 907.185 |
| metric tonnes | pounds | 2204.62 |
| metric tonnes | US tons | 1.10231 |
Three of these are worth memorizing. To convert pounds to kilograms, divide by 2.2 (close enough for casual use; exact is 2.20462). To convert kilograms to pounds, multiply by 2.2. To go between ounces and grams, multiply ounces by 28 or grams by 0.035 — recipes commonly use both, so the conversion comes up often.
Body weight reference table
| Pounds | Kilograms | Stone (UK) |
|---|---|---|
| 100 lb | 45.4 kg | 7 st 2 lb |
| 120 lb | 54.4 kg | 8 st 8 lb |
| 140 lb | 63.5 kg | 10 st 0 lb |
| 150 lb | 68.0 kg | 10 st 10 lb |
| 160 lb | 72.6 kg | 11 st 6 lb |
| 165 lb | 74.8 kg | 11 st 11 lb |
| 180 lb | 81.6 kg | 12 st 12 lb |
| 200 lb | 90.7 kg | 14 st 4 lb |
| 220 lb | 99.8 kg | 15 st 10 lb |
| 250 lb | 113.4 kg | 17 st 12 lb |
About stone — a British peculiarity
Stone is one of those units that almost no English speaker uses except for one specific purpose: telling someone their body weight in the UK and Ireland. A British person weighing 76 kg would say "I'm 12 stone" — or, with more precision, "12 stone 0," or "11 stone 13" if pressed.
One stone is 14 pounds. The number 14 isn't arbitrary; it has historical roots in trade weights for agricultural goods. The unit was standardized in the UK by the Weights and Measures Act of 1985, which kept stone for personal weight even as most other imperial units were phased out for official purposes.
Why 14 pounds rather than 10? Because stone is a "convenient" body-weight unit. Most adult weights land between 8 and 20 stone, which is a small range of whole numbers. Compare with kilograms (40–150) or pounds (90–330). Stone gives you a tidy number with a single decimal — or a stone-and-pound pair — for almost any adult body weight. The format is "X stone Y" where Y is 0–13. After 13 lb you hit the next stone.
UK scales typically display both stone and kilograms. Australian and New Zealand scales display kilograms. American scales display pounds. The Weight Converter handles all three so you can move between them without arithmetic.
The US ton vs the metric tonne
One of the rare cases where the spelling difference actually matters. The US "ton" (sometimes called "short ton") is 2000 pounds, or about 907 kg. The metric "tonne" (or "metric ton," with a space) is 1000 kg, or about 2205 pounds. The UK historically used a "long ton" of 2240 pounds (about 1016 kg), now mostly obsolete.
The difference is about 10% — large enough that confusing the two on a shipping label can cost real money. When in doubt: spell out "metric tonne" or "1000 kg" if that's what you mean, and "2000 pound ton" or "US ton" otherwise. Most international trade defaults to metric tonnes; most US domestic freight defaults to US tons.
Engineering papers usually write "Mg" (megagram) when they mean a metric tonne to avoid any ambiguity — that's 1,000,000 grams, which is exactly 1000 kg, which is exactly one metric tonne. The Mg notation is rare outside technical writing.
Recipes and the gram-ounce trade-off
One specific case where the converter sees heavy use: recipes. American cookbooks use ounces (oz) and pounds. European cookbooks use grams (g) and kilograms. A US recipe that calls for "8 oz of butter" is 226.8 g — not a tidy number, which is why metric-using bakers either round to 225 g (about 1% off) or buy a 250 g pack and use 90% of it.
The conversion in the kitchen direction:
- 1 ounce = 28.35 g, or about 28 g for quick mental math
- 1 pound = 453.6 g, or about half a kilo
- 4 oz (a stick of US butter) = 113.4 g
- 16 oz (1 lb of meat) = 453.6 g
A point worth noting: ounces in recipes are weight ounces. The "fluid ounce" used for liquids in US recipes is a volume measurement, and the two only roughly correspond for water (1 fl oz of water weighs about 1 weight oz, but for any other ingredient the relationship is different). The Weight Converter handles weight ounces. For fluid ounces, use a volume tool like the ML to OZ Converter.
Weighing at high precision
The conversions in this tool are exact in their math but limited by the precision of your input. If a scale reads "165.0 lb," the actual weight is somewhere between 164.95 and 165.05 lb — a range of about 0.045 kg when converted. Most digital bathroom scales are accurate to ±0.1 kg or ±0.2 lb, which is fine for daily use but worse than the precision the converter will display.
For higher-precision applications:
- A kitchen scale typically resolves to 1 g and is accurate to about ±2 g across its range.
- A jewelry scale resolves to 0.01 g (or 0.1 grain for traditional units).
- An analytical balance in a chemistry lab resolves to 0.0001 g.
The converter will give you as many decimal places as the math produces, but trust your inputs. If you measured 100.0 g on a scale that's ±2 g, don't report 3.5274 oz as if the 0.0004 oz matters.
Related conversions
- Length Converter — meters, feet, inches, miles. The other big metric/imperial mismatch.
- ML to OZ Converter — volume in milliliters and fluid ounces. Different from weight ounces; recipes mix the two up.
- Grams to Ounces — single-purpose tool for the most common kitchen conversion.
- OZ to LBS — for converting cooked-meat weights or larger imperial measurements.
- BMI Calculator — pairs your weight conversion with height to give the standard health metric.
Frequently asked questions
Is 1 pound exactly 0.45359237 kg or is that rounded?
Exactly. The international yard and pound agreement of 1959 defined one pound as exactly 0.45359237 kilograms — a definition, not a measurement. Every imperial weight unit (ounce, stone, ton) derives from that definition. Before 1959 the US pound and the UK pound differed by a tiny amount; the agreement merged them.
How many kilograms is 165 pounds?
74.84 kg. The exact math is 165 × 0.45359237 = 74.84 kg. The quick mental version is to divide by 2.2, which gives 75.0 — close enough for most purposes, off by 0.16 kg.
What's the easiest way to convert pounds to kilograms in my head?
Divide by 2.2. The exact conversion is divide by 2.20462, so the rule-of-thumb is about 0.2% too high — you get a slightly larger kilogram number than the exact value. For 100 lb, "divide by 2.2" gives 45.45 kg, when the exact is 45.36 kg. Good enough for travel and conversation; switch to the exact value for medical or technical use.
What is "stone" and why do the British still use it?
Stone is a British weight unit equal to 14 pounds, used almost exclusively for body weight. A 12-stone person weighs 168 lb or about 76 kg. It survived the UK's gradual switch to metric because it's convenient for body weight — adult weights fall in a small, tidy range when measured in stone — and because the cultural attachment is strong. UK scales typically display kilograms and stone together so people can read either.
What's the difference between a US ton and a metric tonne?
About 10%. A US ton is 2000 pounds, or 907.185 kg. A metric tonne is 1000 kg, or 2204.62 lb. They look almost identical in writing, which is exactly why shipping documents typically spell out which one they mean. UK shipping historically used a "long ton" of 2240 lb (1016 kg), now mostly retired.
How accurate is "1 kg = 2.2 lb"?
About 0.2% off. The exact conversion is 1 kg = 2.20462 lb. For a 70 kg person, the rule-of-thumb gives 154 lb when the exact is 154.32 lb. For everyday conversation, fine. For prescriptions, baggage allowances near a limit, or anything legally binding, use the exact value.
Why are kitchen ounces and fluid ounces different?
Because one measures weight and the other measures volume. A US fluid ounce is 29.57 ml — a unit of volume. A weight ounce is 28.35 g — a unit of mass. They happen to be roughly equivalent for water (because water is close to 1 g/ml), which is why US recipes sometimes treat them as interchangeable. For any ingredient denser than water (honey, syrup) or less dense (oil, flour by volume), the two are noticeably different. The Weight Converter handles weight ounces.
What's the smallest weight the converter handles?
Milligrams. For medication doses, jewelry, and lab work, milligrams (mg) and grams (g) are usually the right range. The converter covers up to metric tonnes on the heavy end — enough for shipping pallets and freight weights. For micrograms (μg) used in pharmacology or pesticide labels, a scientific calculator with SI prefix support is a better fit.