Weight Converter

Includes stones and pounds, which almost no international converter handles properly.

The exact factors

Three different “tons” is a genuine hazard in shipping and engineering. The metric tonne and the US short ton differ by about 10 %, which is easily enough to matter when specifying a crane or costing freight.

Stones, and where they are still used

The stone survives almost exclusively in the UK and Ireland, and almost exclusively for body weight. It is 14 pounds — a figure that looks arbitrary because it is: medieval stones varied by commodity, from 8 pounds for beef to 32 for hemp, and the 14-pound wool stone was standardised in 1835.

To convert, divide pounds by 14 for the whole stones and take the remainder as the loose pounds. 165 lb is 11 stones (154 lb) remainder 11, so 11 st 11 lb. The awkward part is that writing 11.8 stones means 11 stones and 0.8 × 14 = 11.2 pounds, not 11 stones 8 pounds — a common source of confusion.

Mass, weight and the redefined kilogram

Strictly, mass is the amount of matter in an object and weight is the force gravity exerts on it. A 75 kg person has the same mass on the Moon but weighs about one sixth as much. In everyday use “weight” means mass, and bathroom scales are calibrated for Earth's gravity.

The kilogram was the last SI unit defined by a physical artefact — a platinum-iridium cylinder held near Paris since 1889. That cylinder was drifting relative to its official copies by a few tens of micrograms, which meant the world's definition of mass was quietly changing. In May 2019 the kilogram was redefined in terms of the Planck constant, fixed at exactly 6.62607015 × 10⁻³⁴ joule seconds. No SI unit now depends on a physical object.