Understanding Weight Units

The kilogram (kg) is the SI base unit of mass and the only base unit still defined by a physical artefact — until 2019, when the International Prototype Kilogram, a platinum-iridium cylinder kept in a vault near Paris, was retired. Today the kilogram is defined via the Planck constant, tying it to fundamental physics rather than a lump of metal. The pound is defined exactly as 0.453592 kg, a fixed relationship set in 1959 by international agreement.

Common Weight References

A US nickel coin weighs exactly 5 grams — handy for calibrating a postal scale in a pinch. One litre of pure water at 4 °C weighs exactly 1 kg, which is no coincidence: the original kilogram was defined that way. A typical 70 kg adult is 154 lb or 11 stone. At the far end of the scale, an African elephant averages around 6,000 kg — roughly the weight of four average cars.

Weight vs. Mass

Technically, weight is a force measured in Newtons, while mass is the quantity of matter measured in kilograms. When someone says they "weigh 70 kg," they mean their mass is 70 kg. On the Moon, where gravity is about one-sixth of Earth's, you'd weigh roughly 11.5 kg-force but your mass would remain exactly 70 kg. Your bathroom scale measures the force of gravity pulling you down and then displays the equivalent mass — it only gives the right number if you're standing on Earth at roughly sea level.

Why the UK Still Uses Stone

One stone equals 14 pounds, or about 6.35 kg. It's a medieval trade unit that survived British metrication almost entirely because of one use case: describing body weight. Ask someone their weight in a British pub and they'll answer in stone and pounds. Ask the same question in Germany or France and you'll get kilograms; in the US, pounds. Outside of describing a person's heft, stone is almost never used — you won't see it on food packaging, in engineering, or in sport.