Imperial Weight Converter

What this converter does

It converts between the avoirdupois chain in any combination: grain, dram, ounce, pound, stone, US hundredweight, imperial hundredweight, US short ton and imperial long ton. The US and imperial variants appear as separate entries throughout, because they are different masses that share a name and keeping them apart is the whole point.

How the chain fits together

Sixteen drams make an ounce. Sixteen ounces make a pound. Fourteen pounds make a stone. Above that the systems diverge: the US hundredweight is 100 lb while the imperial one is 112 lb, so the US short ton is 2,000 lb and the imperial long ton is 2,240. Underneath everything, the pound is exactly 7,000 grains, which ties the entire chain to a single smallest unit and is what makes every conversion within it exact.

Why two hundredweights and two tons

Because Britain kept the traditional 112-pound hundredweight, which is eight stone of fourteen pounds, while the United States rationalised the unit to a decimal 100 pounds. Twenty hundredweight make a ton in both systems, so that single divergence propagates upward and produces two tons differing by exactly twelve percent. Every difference between American and British weights above the pound traces back to that one decision, which is a remarkably tidy piece of history for a system otherwise built by accretion.

Why all these conversions are exact

Every unit in the chain is a whole-number multiple of the grain, and the grain is exactly one seven-thousandth of the international pound, which is exactly 0.45359237 kilograms. So conversions within the chain are exact by construction, and conversions out to metric are exact by the 1959 agreement. Nothing here is measured, which means any disagreement between two correct converters is a display rounding rather than a difference of substance.

The dram

One sixteenth of an ounce, appearing in older shipping papers, pharmacopoeias and some traditional recipes. It has no dedicated page in this cluster because the dominant searches for dram concern fluid drams and whisky measures, which are volumes rather than masses — building a mass-dram page would attract readers looking for something else entirely. It is included here as a unit so that documents using it can be converted.

Which units are still in use

Ounces, pounds and stones in everyday British and American life. Grains in ammunition, archery and legacy pharmacy. The US short ton in American commerce. The imperial long ton in shipping displacement and pre-metric British documents. The dram and both hundredweights are largely historical, surviving in trade records and in a few agricultural pricing conventions — US milk and livestock are still quoted per hundredweight.

Troy units are not here

Deliberately. Troy ounces, troy pounds and pennyweights belong to a separate system with different sizes: the troy ounce is heavier than the avoirdupois ounce while the troy pound is lighter, which is exactly the sort of confusion that mixing them into one converter would create. They have their own pages in this cluster, with their own constants and their own worked examples.

Reading an unlabelled ton or hundredweight

Geography and date usually settle it. American sources mean the short ton and the 100-pound hundredweight; British sources before metrication mean the long ton and the 112-pound hundredweight; anything modern and international means the metric tonne, which is a third unit again. Where a document gives two related figures — a ton and a hundredweight count, say — the ratio identifies the system, since twenty hundredweight make a ton in both.

Common mistakes

Reading a bare ton as the wrong one, which costs up to twelve percent. Applying sixteen to a stone conversion, where the factor is fourteen. Mixing troy and avoirdupois. And assuming a hundredweight is a hundred of something obvious — it is a hundred pounds in one system and 112 in the other, and neither is a hundred kilograms, which is the quintal.

Benchmarks

1 lb is 7,000 gr, 256 dr and 16 oz. 1 stone is 14 lb. 1 US cwt is 100 lb; 1 imperial cwt is 112 lb. 1 short ton is 2,000 lb; 1 long ton is 2,240 lb. In metric: a pound is 0.4536 kg, a stone 6.35 kg, a short ton 907.18 kg and a long ton 1,016.05 kg.

Accuracy and limits

Exact factors throughout, exact decimal arithmetic, display-only rounding, and a parser that refuses ambiguous input with an explanation. Avoirdupois units only — troy weights and metric units have their own converters, kept separate by design.

Why the system looks arbitrary

Because it was assembled rather than designed. Sixteen ounces to the pound suits a balance, which halves naturally; fourteen pounds to the stone came from the wool trade; 112 pounds to the hundredweight came from eight of those stones; 2,240 pounds to the ton from twenty hundredweight. Each step made sense in its own trade, and the whole only became a system afterwards. The metric system was designed at once and looks it; the avoirdupois system accumulated and looks like that too.

The 1959 agreement

Before 1959 the US and UK pounds differed by about one part in ten million, which sounds negligible and mattered enormously to metrology and precision engineering. The International Yard and Pound Agreement fixed the pound at exactly 0.45359237 kilograms for every signatory, which is what makes every conversion in this cluster exact rather than approximate. It also means the entire imperial system is now defined metrically — every unit on this page ultimately rests on the kilogram, including the ones that look least metric.

Reading an old trade document

Weights in older British commercial papers frequently appear as compound figures — 3 tons 4 cwt 2 qr 7 lb — where qr is the quarter, two stone or 28 pounds. Resolving such a figure means converting everything to pounds and summing: 3 times 2,240, plus 4 times 112, plus 2 times 28, plus 7. Doing it that way once is far more reliable than converting each component to metric separately and adding, which multiplies the opportunities for an error without improving the result.

Using this page to audit another tool

Because every constant here is exact, the converter doubles as a reference for checking a spreadsheet, a shipping system or a piece of firmware. Useful anchors: 1 lb must give exactly 16 oz and exactly 7,000 gr; 1 stone exactly 14 lb; 1 short ton exactly 2,000 lb; 1 long ton exactly 2,240 lb; 1 imperial cwt exactly 112 lb. A tool that returns 13.999999999999998 for the stone test is doing binary floating point and rounding late, which is harmless at two decimals and tells you exactly where its digits stop being real.