Weight, mass and what the pound really is
In everyday speech, weight means how heavy something is, and it is measured in kilograms or pounds. In physics, mass is the amount of matter and weight is the pull of gravity on it, so an object weighs less on the Moon but keeps the same mass. Kilograms, grams and pounds are units of mass, and this converter converts mass. Gravity plays no part, which is exactly what you want when comparing a package in kilograms with a scale in pounds.
The kilogram is the base unit. Since 1959 the avoirdupois pound has been defined as exactly 0.45359237 kg, and the ounce as one sixteenth of a pound, which makes 1 oz exactly 28.349523125 g. Those are definitions, so the conversions are exact and the converter does not use rounded factors.
Same names, different sizes
Some unit names hide more than one size. The converter lists each one separately, so you can pick the one you mean:
- Ounce (avoirdupois) is 1/16 pound, 28.35 g. It is the one on food labels and kitchen scales.
- Troy ounce is 31.1034768 g, about 1.097 ordinary ounces. Gold, silver and platinum are priced per troy ounce.
- Metric ton (tonne) is 1,000 kg, about 2,204.6 lb.
- US ton (short ton) is 2,000 lb, 907.185 kg.
- UK ton (long ton) is 2,240 lb, 1,016.047 kg.
- Carat is 200 mg and measures gemstones. It is not the same as the karat that gives the purity of gold, such as 18 karat.
Quick estimates that are close enough
- Kilograms to pounds: double it and add 10 percent. 50 kg is 100 + 10 = 110 lb (really 110.2). The error is about 0.2 percent.
- Pounds to kilograms: halve it and take off 10 percent. 200 lb is 100 less 10 = 90 kg (really 90.7). The error is about 0.8 percent.
- Ounces to grams: multiply by 28 for a rough answer, 28.35 for a good one. 8 oz is 226.8 g.
- Stone to kilograms: multiply by 6.35. 10 st is 63.5 kg.
For postage, medicine doses, recipes or anything that has to be right, use the exact answer from the converter.