Understanding Weight Measurements — kg, Pounds, Stone and More

Weight is measured differently around the world, and the differences matter more than you might think. Here's a clear guide to all the main weight units and how to think about them.

The kilogram (kg) is the SI base unit of mass and the standard used in science, medicine and most of the world for everyday weighing. The kilogram was originally defined as the mass of one litre of water at its maximum density. Since 2019 it has been redefined in terms of Planck's constant — a quantum mechanical constant — making it more precise and stable than any physical prototype could be.

Pounds (lb) are the primary weight unit in the United States and are still used alongside kilograms in the United Kingdom. One pound = 0.453592 kilograms. The abbreviation "lb" comes from the Latin word "libra" — the same root as the pound sterling (£) currency symbol, which also derives from a pound weight of silver.

Stone is a unit used almost exclusively in the UK and Ireland for measuring human body weight. One stone = 14 pounds = 6.35029 kilograms. The stone has no equivalent in other measurement systems and is one of the most confusing units for non-British people to interpret — someone who weighs 11 stone 4 pounds weighs 71.67 kilograms or 158 pounds.

Tons come in three varieties, which cause frequent confusion in international contexts. A metric tonne (t) = 1,000 kg. A US short ton = 2,000 pounds = 907.185 kg. A UK long ton = 2,240 pounds = 1,016.047 kg. When someone mentions "tons" without specifying, the context usually indicates which one — the UK tends to use metric tonnes now, while the US uses short tons, and long tons appear mostly in historical texts.