Weight Converter

Where the units of weight come from

The kilogram is the SI unit of mass. Until 2019 it was the mass of a platinum-iridium cylinder kept near Paris; it is now defined by fixing the value of the Planck constant, so it can be realized in any well-equipped laboratory. Grams, milligrams and tonnes are powers of ten of it: a tonne is 1,000 kg.

The everyday US and British units belong to the avoirdupois system. Since the 1959 international agreement the pound is exactly 0.45359237 kg, and the rest is defined from it:

• 1 ounce = 1/16 pound = 28.349523125 g
• 1 stone = 14 pounds = 6.35029318 kg (used for body weight in the UK and Ireland)
• 1 US short ton = 2,000 pounds; 1 long ton (the British ton) = 2,240 pounds

The troy ounce is a different, older unit that survives for gold, silver, platinum and palladium. It is 480 grains of exactly 64.79891 mg each, or 31.1034768 g. The avoirdupois pound is 7,000 of the same grains, which is how the two systems are tied together.

The conversion formula

Each unit has a fixed size in kilograms, and converting means multiplying by the ratio of the two sizes:

value

the weight you enter

size

one unit in kilograms, e.g. 0.45359237 kg for a pound and 1 kg for a kilogram

Pounds to kilograms is a single multiplication by 0.45359237. Kilograms to pounds is a division by the same number: the familiar 2.2046… pounds per kilogram is itself a rounded result of 1 ÷ 0.45359237, so the calculator divides by the defined figure and the working shows it. Results that end are shown in full; others are rounded to 10 significant digits.

Worked examples

1 kilogram to pounds. 1 ÷ 0.45359237 = 2.204622622 lb. For 70 kg the answer is 70 ÷ 0.45359237 = 154.3235835 lb.

11 stone 4 pounds to kilograms. Turn it into pounds first: 11 × 14 + 4 = 158 lb. Then 158 × 0.45359237 = 71.66759446 kg, an exact result because both numbers end.

1 troy ounce of gold in grams and ordinary ounces. It is exactly 31.1034768 g, and 31.1034768 ÷ 28.349523125 = 1.097142857 avoirdupois ounces, so a troy ounce is about 9.7% heavier than the ounce on a kitchen scale.

Common weight conversions

Mass, weight and other traps

• Weight usually means mass. Strictly, weight is the force gravity puts on an object and is measured in newtons; mass is the amount of matter, measured in kilograms. In shops, recipes and medicine, “weight” means mass, and NIST notes that this everyday usage is the norm. A bathroom scale measures force but is calibrated to show mass, which is why it would read low on the Moon while your mass stayed the same.
• There are three tons. A US short ton is 2,000 lb (NIST’s required abbreviation is “tn”), a British long ton is 2,240 lb, and a metric tonne (“t”) is 1,000 kg, about 2,204.6 lb. Shipping, freight and vehicle specifications can use any of them, so check which one is meant.
• Troy and avoirdupois ounces differ. Precious-metal prices are per troy ounce (31.1 g), not per ordinary ounce (28.3 g). Oddly, the troy pound of 12 troy ounces (373.2417216 g) is lighter than the ordinary pound.
• Fluid ounces measure volume, not weight. A US fluid ounce of water weighs about 1.04 oz (roughly 29.5 g at room temperature); for oil or flour the weight is different again. For liquid measures use the volume converter.