Article — Weight Converter
Weight Converter: Kilograms to Pounds, Ounces, Stones, and Tons
A weight converter translates kilograms into any of eight common weight units: pounds (2.20462 lb per kg), ounces (35.27 oz), grams (1000 g), stones (0.15747 st), milligrams, metric tonnes, US short tons, and UK long tons. All factors are exact international treaty values, not measurements.
Since 2019 the kilogram itself has been defined by the Planck constant, replacing the platinum cylinder that used to live in a Paris vault. Every other unit of weight in everyday use derives from the kilogram through fixed ratios. There is no measurement uncertainty in any of these conversions — only arithmetic.
What a weight converter does
A weight converter takes a value in one mass unit and expresses it in another. The math is multiplication: target = source × factor. The factor for each pair is a known constant, recorded in standards documents from NIST, BIPM, and ISO.
What makes a good converter is not the formula but the unit coverage and the precision of the factors. The pound is defined as exactly 0.45359237 kg, so the kilogram-to-pound factor is exactly 1 / 0.45359237 = 2.20462262184878. Many calculators round this to 2.2046 or even 2.2, introducing small errors that compound at industrial scale.
The international yard and pound agreement of 1959 ended a half-century of small but real disagreements between US and UK definitions of the pound. Before the treaty, a US pound was about 0.06 milligrams lighter than a UK pound (453.5924277 g vs 453.59237 g). The treaty fixed both at exactly 0.45359237 kg, a value still in use today.
Weight conversion formulas
The cleanest way to handle multi-unit conversion is to route everything through kilograms. Convert the source to kilograms, then convert kilograms to the target. The two steps require only two constants per unit.
1 kg = 2.20462 lb 1 kg = 35.274 oz1 kg = 1000 g 1 kg = 0.15747 st1 short ton = 907.185 kg 1 long ton = 1016.05 kg1 metric tonne = 1000 kg 1 stone = 6.35029 kgNotice that the metric units (g, mg, tonne) all relate to the kg by powers of 10. That is the appeal of metric arithmetic: shift the decimal point and you have the answer. The Imperial units (lb, oz, stone, short and long tons) each carry their own irrational factor, locked in by treaty rather than designed for round-number conversion.
Kilograms to pounds (the most common conversion)
Kilograms to pounds is the single most-searched weight converter query. Multiply kilograms by 2.20462 to get pounds. The mental shortcut is to double the kilograms and add 10% — 70 kg × 2 = 140, plus 10% = 154 lb. The true answer is 154.32 lb, so the shortcut is correct to within 0.2%.
Body weight is the typical use case. A 70 kg adult weighs 154 lb. A 100 kg adult weighs 220 lb. Going the other way, the rough rule is pounds ÷ 2 then subtract 10% — 200 lb ÷ 2 = 100, minus 10% = 90, true 90.7 kg.
UK stones and the weight converter
British and Irish adults often give body weight in stones plus pounds, not in kilograms. One stone equals 14 pounds equals 6.35029 kg. So 11 stone 4 pounds is (11 × 14 + 4) = 158 lb = 71.7 kg.
The stone is no longer an official unit in the UK — only in horse racing and a few traditional commercial contexts. But it survives in casual conversation. Doctors prescribing weight loss in stones are common; bathroom scales sold in the UK still show stones alongside kilograms.
The three tons: short, long, metric
"Ton" means three different things depending on context. The short ton, used in the US and Canada, is 2000 pounds (907.185 kg). The metric tonne is 1000 kg (2204.62 lb). The long ton, traditional in the UK and still in some shipping, is 2240 pounds (1016.05 kg).
The metric tonne is the international standard and the default in most modern contexts. Bulk shipping contracts written in English usually say "metric tonne" or "tonne" explicitly to avoid confusion. A figure quoted as just "ton" should always be clarified — the difference between short ton and metric tonne is 9.3%, which is large enough to matter for commercial deliveries.
The everyday avoirdupois ounce (28.349 g) is used for food, postage, and almost everything else. The troy ounce (31.103 g), about 10% heavier, is reserved for precious metals — gold, silver, platinum. A 1-oz gold coin weighs more than a 1-oz can of soup.
Mass vs. weight — the physics distinction
Strictly, mass is a property of matter; weight is the force gravity exerts on that matter. Mass is measured in kilograms; weight is measured in newtons. A 70 kg astronaut has the same 70 kg of mass on Earth, on the Moon, and in deep space, but their weight changes dramatically.
In everyday use, mass and weight are treated as synonyms. Scales report "weight" in kilograms or pounds, which are mass units. The physics distinction matters only in scientific contexts (orbital mechanics, low-gravity experiments) or in very precise engineering where the local value of gravitational acceleration affects calibration. For all consumer purposes, a weight converter is a mass converter, regardless of what the input label says.
Common weight converter mistakes
Mixing avoirdupois and troy units is the first trap. A 1-ounce silver coin is a troy ounce (31.1 g), not an avoirdupois ounce (28.35 g). The difference is 9.7%, which matters when buying bullion.
The second trap is the ton-type confusion. Spec sheets, freight contracts, and government regulations all use "ton" without always clarifying which one. Read carefully for "short," "long," "metric," or the symbol t (metric tonne).
The third trap is rounding the kg-to-lb factor too aggressively. Using 2.2 instead of 2.20462 introduces a 0.2% error that compounds for industrial weights. At 10 metric tonnes, the difference is 22 kg — a noticeable amount on a shipping invoice.
- 1 kg = 2.20462 lb (exact)
- 1 lb = 0.45359237 kg (treaty value, 1959)
- 1 stone = 14 lb = 6.35029 kg
- 1 oz (avoirdupois) = 28.349 g
- 1 oz (troy) = 31.103 g (precious metals only)
- 1 short ton = 2000 lb = 907.185 kg
- 1 metric tonne = 1000 kg = 2204.62 lb
- 1 long ton = 2240 lb = 1016.05 kg
For mental math between kg and lb, remember "doubles plus 10%" going up and "halves minus 10%" going down. The trick is accurate to within 0.2%, good enough for groceries, gym plates, and most medical contexts. Switch to the exact factor only when the result is destined for a paid invoice.