Weight Converter

Converting mass and weight

This converts between the metric and imperial units used for mass in everyday life, shipping, cooking and science. All conversions run through the kilogram, which since 2019 has been defined by fixing the Planck constant rather than by a metal cylinder in a vault outside Paris.

The pound-to-kilogram figure is exact, not rounded. Since 1959 the international avoirdupois pound has been defined as exactly 0.45359237 kg by agreement between the English-speaking countries, so any imprecision in a conversion comes from rounding the output, never from the factor.

Common conversions

Three different tons

This causes genuine and expensive confusion in freight and construction, because the three units differ by up to 12%:

• Metric tonne — 1,000 kg, about 2,205 lb. Standard in most of the world and in international shipping.
• US short ton — 2,000 lb, about 907 kg. The default meaning of "ton" in the United States.
• UK long ton — 2,240 lb, about 1,016 kg. Historically British, still used in some shipping contexts.

A quoted "50 tons" therefore means 45,359 kg, 50,000 kg or 50,802 kg depending on who wrote it. On any commercial document, check which is meant before you sign.

Stone, and where it is used

The stone equals 14 pounds and survives almost exclusively in Britain and Ireland for body weight. A person described as "11 stone 4" weighs 158 lb or about 71.7 kg. When converting, remember the second number is pounds out of 14, not a decimal — 11 stone 7 is 11.5 stone, not 11.7.

Mass and weight are not the same thing

Strictly, mass is the amount of matter in an object and does not change with location; weight is the force gravity exerts on that mass and does. An object of 70 kg mass weighs about 686 newtons on Earth and about 114 newtons on the Moon, while its mass stays 70 kg throughout.

In ordinary use the distinction is ignored, because everything is weighed on the same planet and a kilogram of mass reliably corresponds to a fixed weight. It matters in physics, in engineering calculations involving force, and occasionally in trade, where the pound-force and pound-mass are distinct units that share a name. This tool converts mass, which is what a kitchen or bathroom scale is calibrated to report.

Cooking: mass beats volume

Recipes written in cups measure volume, and the mass of a cup depends entirely on what is in it and how it was packed. A cup of all-purpose flour ranges from about 120 g when spooned and levelled to 150 g when scooped and compacted — a 25% difference that ruins baking. A cup of granulated sugar is around 200 g, and a cup of water is 236 g.

This is why serious baking recipes give weights. If you are converting a volume recipe, look up the specific ingredient's density rather than applying a single factor, and weigh where you can. There is no general conversion between cups and grams, only per-ingredient ones.

Precision and rounding

For most purposes two decimal places is more than enough. Converting back and forth repeatedly compounds rounding error, so convert once from your original figure rather than chaining conversions. For trade, medicine or engineering, work to the precision your standard specifies — a dosage calculation rounded carelessly is a real hazard, and shipping weights near a tariff threshold can cost money on a rounding decision.

Frequently asked questions