How mass units are defined
The kilogram is the SI base unit of mass. Since May 2019 it is no longer a metal cylinder in a vault outside Paris: it is defined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ J s. Grams, milligrams, micrograms and tonnes are decimal multiples of it, so those conversions are exact by construction.
The imperial units are defined against the kilogram. The 1959 international yard and pound agreement fixed the avoirdupois pound at exactly 0.45359237 kg. From that one number the rest follows: an ounce is 1/16 lb = 28.349523125 g, a stone is 14 lb = 6.35029318 kg, and a grain is 1/7000 lb = 64.79891 mg.
Three different tons are in daily use: the metric tonne (1,000 kg), the US short ton (2,000 lb = 907.18474 kg) and the UK long ton (2,240 lb = 1,016.0469088 kg). Freight quotes, emissions figures and material orders often say only “ton”, so the unit table lists all three rather than picking one.
Converting by hand
Kilograms and pounds: multiply kg by 2.20462262 for pounds, or multiply pounds by 0.45359237 for kilograms. 70 kg × 2.20462262 = 154.32 lb; 165 lb × 0.45359237 = 74.84 kg. The everyday shortcut “double it and add ten percent” gives 154 for 70 kg, which is close enough for a suitcase and not for a prescription.
Stones and pounds: divide the kilograms by 6.35029318 to get stones, then multiply the fractional part by 14. 75 kg ÷ 6.35029318 = 11.81 st, and 0.81 × 14 = 11.3 lb, so 75 kg is 11 st 11 lb. British medical notes and scales still use this form.
Kitchen quantities: 1 oz = 28.3495 g, so a 500 g bag of flour is 17.64 oz and a US recipe asking for 8 oz of butter needs 226.8 g. A baby weighing 8 lb 6 oz is 8.375 lb × 0.45359237 = 3.799 kg — convert the ounces to a fraction of a pound before multiplying.
Quick reference
- 1 kg = 2.20462 lb · 1 lb = 453.592 g · 1 oz = 28.3495 g · 1 g = 0.035274 oz
- 50 kg = 110.23 lb · 60 kg = 132.28 lb · 70 kg = 154.32 lb · 80 kg = 176.37 lb
- 1 st = 6.35029 kg · 10 st = 63.50 kg · 12 st = 76.20 kg · 14 st = 88.90 kg
- 1 tonne = 1,000 kg = 2,204.62 lb · 1 US ton = 907.185 kg · 1 UK ton = 1,016.047 kg
- 1 troy oz = 31.1035 g (gold, silver) vs 1 avoirdupois oz = 28.3495 g
- Airline checked bag 23 kg = 50.7 lb · barbell plate 20 kg = 44.09 lb vs 45 lb = 20.41 kg
Avoirdupois, troy and metric: three kinds of ounce
The ounce in this converter is the avoirdupois ounce, 28.349523125 g — the one used for food, post and body weight. Precious metals are traded in troy ounces of 31.1034768 g, about 10% heavier. When a headline quotes “gold at $2,400 an ounce” it means a troy ounce, so converting with 28.35 g understates the metal by a tenth.
The pounds differ too. A troy pound is 12 troy ounces (373.24 g), not 16, so it is lighter than an avoirdupois pound despite each of its ounces being heavier. Apothecaries’ weights, still seen in old pharmacy texts, use the troy ounce with yet another subdivision.
A fluid ounce is not a mass at all. It measures volume — 29.5735 mL in the US, 28.4131 mL in the UK — and only coincides with the avoirdupois ounce for water-like densities. Recipes that say “8 oz” of a liquid mean fluid ounces; “8 oz” of cheese means mass.
Common mistakes
Mass and weight are not the same quantity. A kilogram measures mass; the force it produces under gravity is 9.80665 newtons. Engineering documents that say “kgf” or “lbf” mean force, and 1 lbf = 4.4482216 N. Everything on this page is mass, which is what scales, invoices and shipping labels use.
Guessing which ton. A 12% gap separates a US short ton from a UK long ton, and the metric tonne sits between them. On CO₂ figures, freight rates and steel orders the wrong choice moves the number by more than most people’s margin, so check whether the source is American, British or metric.
Rounding 2.2 for 2.20462. On 5 kg the error is 11 g and nobody notices; on a 500 kg pallet it is 2.3 kg, which is enough to cross a shipping bracket. Convert once with the full factor and round at the end.
Traditional units are not in the table. In Korea a don (돈) is 3.75 g and is how jewellery is sold, a geun (근) is 600 g for meat but 375 g for vegetables and fruit, and a gwan (관) is 3.75 kg. Convert to grams first, then use the table.
Where these conversions matter
Shipping and travel. Airlines publish 23 kg (50.7 lb) checked limits and 7 kg (15.4 lb) carry-on limits, and couriers bill on the greater of actual and volumetric weight. A parcel quoted in pounds by a US carrier and in kilograms by a local one is the same box priced two ways.
Cooking and nutrition. American recipes use ounces and pounds, European ones grams, and nutrition labels differ per 100 g versus per serving. Converting the whole recipe at once, rather than each line as you go, keeps the ratios intact.
Fitness and health. Gym plates come in 20 kg and 45 lb versions that are 0.41 kg apart, so a “225 lb” bench is 102.06 kg but a metric gym’s nearest equivalent is 100 kg. Body weight in kilograms is also what a BMI formula expects.