Multi-Scale Weight Converter

Weight - or more precisely mass - was among the first quantities ever standardised. Sumerian tablets from 3000 BCE record a barley-grain weight standard; Egypt had the deben at around 91 g by the New Kingdom. Mass standards were always tied to commerce: traders needed to know whether the silver they were exchanging matched the silver they had been promised, and rulers needed to tax it. Every 'weight' in modern English carries traces of those ancient grain-and-coin standards.

The pound has multiple ancestors. The Roman libra (~327 g) gave us both the 'lb' abbreviation and the £ symbol for currency. The medieval Tower pound, the merchant pound, the troy pound, and the avoirdupois pound coexisted in Britain for centuries, each used for different goods. The avoirdupois pound (453.592 g) was made standard by the 1878 Weights and Measures Act and is the pound used today on bathroom scales and at the supermarket.

The troy system, named after the medieval French trading town of Troyes, survives only for precious metals and gunpowder. A troy ounce is 31.1035 g - heavier than an avoirdupois ounce (28.35 g). A troy pound (373.24 g) is divided into 12 troy ounces, not 16. Get this wrong on a gold transaction and you are losing roughly 10% by mass. Every bullion dealer in the world quotes spot prices per troy ounce.

The carat for gemstones was standardised internationally in 1907 at exactly 200 milligrams. The name comes from the carob seed, which medieval Middle Eastern jewellers used because the seeds' weights are extraordinarily uniform (~200 mg each). A 1-carat round brilliant diamond is about 6.5 mm across. The 'karat' spelling - 24-karat gold is pure - is a separate fineness scale unrelated to mass.

The metric system gave the world a single coherent mass unit: the kilogram. From 1889 to 2019 it was defined by Le Grand K, a platinum-iridium cylinder stored under three nested bell jars at the BIPM near Paris. Six official copies and dozens of national prototypes were periodically compared, and small drifts were observed - which is exactly why metrology eventually moved away from artefact definitions.

On 20 May 2019 the kilogram was redefined in terms of the Planck constant h (6.626 070 15 × 10⁻³⁴ J·s). A Kibble balance can now realise the kilogram from electrical measurements rooted in fundamental physics, independent of any physical artefact. Le Grand K is now a museum piece; any laboratory with sufficient precision can in principle realise the kilogram themselves. This is the most profound change in metrology since the metric system itself.

Modern weighing covers an extraordinary dynamic range. Microbalances at NIST resolve individual molecules at femto-gram precision (10⁻¹⁵ g). Truck weighbridges resolve to ±10 kg out of 40 tonnes. Surface gravity differences mean a person weighs about 5 g more at the North Pole than at the equator - hence the careful distinction between 'mass' (intrinsic) and 'weight' (force = mass × g). This calculator gives mass, the quantity that does not change when you change your latitude or planet.