Weight Converter

Mass and weight are not the same thing

Strictly, mass is the amount of matter and weight is the force gravity exerts on it. A 70 kilogram person has the same mass on the Moon and about a sixth of the weight. Everyday usage conflates the two, and these conversions are mass conversions, which is what people actually want. The distinction only matters in physics and in engineering calculations involving force.

Stone is not decimal

One stone is 14 pounds, and British and Irish body weight is conventionally given as stone-and-pounds: 11 stone 4, not 11.29 stone. Converting 158 pounds to 11.29 stone is arithmetically correct and reads as wrong to anyone who uses the unit. The remainder must be expressed in pounds.

Ounces come in two kinds

The avoirdupois ounce, at 28.35 grams, is the one used for food and general weight. The troy ounce, at 31.10 grams, is used for precious metals and is about 10 percent heavier. A price quoted per ounce for gold means troy, and confusing the two produces an error large enough to matter commercially.

The tonne, the ton and the ton

A metric tonne is 1000 kilograms. A US short ton is 2000 pounds, about 907 kilograms. A UK long ton is 2240 pounds, about 1016 kilograms. Three different units, all commonly written as ton, differing by up to 12 percent. Shipping and freight documents are where this causes real disputes, so the unit should always be stated explicitly.

Exact definitions

Since 1959 the international pound is defined as exactly 0.45359237 kilograms, so pound-to-kilogram conversion is exact rather than measured. The kilogram itself has been defined by the Planck constant since 2019, replacing the physical prototype cylinder that had been drifting measurably against its own copies.