An old riddle asks which is heavier, a pound of feathers or a pound of gold. The trick answer is that they weigh the same. The real answer is the feathers. Feathers are weighed in avoirdupois pounds of 453.6 grams, and gold was traditionally weighed in troy pounds of 373.2 grams. Stranger still, an ounce of gold is heavier than an ounce of feathers. Both facts come from two systems of weight that share a name, share a smallest unit, and divide it up differently.
Two Systems Built on the Same Grain
The grain is the unit both systems have in common. It takes its name from a single cereal grain, and today it is defined as exactly 64.79891 milligrams. Avoirdupois, troy and the old apothecaries' system all use the same grain, and they differ only in how many grains they group into an ounce and a pound.
The avoirdupois system, used for nearly everything sold by weight in the US, and in the UK before metrication, puts 7,000 grains in a pound and divides the pound into 16 ounces. That makes an ounce 437.5 grains.
The troy system puts 480 grains in an ounce and 12 ounces in a pound, so a troy pound is 5,760 grains.
Lay the numbers next to each other and the riddle explains itself. The troy ounce, at 480 grains, beats the avoirdupois ounce of 437.5 by about 9.7%. But the troy pound has only twelve ounces where the avoirdupois pound has sixteen, so it ends up about 18% lighter.
The Units in Grains and Grams
Gram values are exact, derived from the grain of 64.79891 mg. The carat is the exception: it was defined directly as 200 mg.
| Unit | System | Grains | Grams |
|---|---|---|---|
| Grain (gr) | All three | 1 | 0.06479891 |
| Carat (ct) | Metric | 3.0865 | 0.2 |
| Pennyweight (dwt) | Troy | 24 | 1.55517384 |
| Ounce (oz) | Avoirdupois | 437.5 | 28.349523125 |
| Troy ounce (oz t) | Troy | 480 | 31.1034768 |
| Troy pound | Troy | 5,760 | 373.2417216 |
| Pound (lb) | Avoirdupois | 7,000 | 453.59237 |
Why Gold Still Uses Troy Ounces
Troy weight is generally thought to take its name from Troyes, a market town in Champagne that hosted some of the great trade fairs of medieval Europe. It became the standard for coins and precious metals in England, while avoirdupois took over everyday trade.
Britain's Weights and Measures Act 1878 cut troy back to a single unit. The troy pound lost its legal standing, and the troy ounce was kept, specifically for gold, silver and other precious metals. That is the arrangement that survives. The LBMA Gold Price, the London benchmark set twice each business day, is quoted in US dollars per troy ounce, and so are the prices on bullion dealers' websites, often written simply as "oz".
The practical consequence is a trap. A spot price of $2,000 per ounce means per troy ounce. Convert a 100 g bar using the kitchen ounce and you get 3.53 ounces, worth $7,055 on paper. The bar is actually 3.22 troy ounces, worth $6,430. The error is just under 10%, in the direction that makes the metal look more valuable than it is.
A kilo bar is 32.15 troy ounces
Bullion is cast in both metric and troy sizes, so you will see 1 kg bars alongside 10 oz and 100 oz bars. 1,000 g ÷ 31.1034768 g gives 32.1507 troy ounces in a kilogram, against 35.27 ordinary ounces. Use the first number when you multiply by a gold price.
Where the Grain Still Turns Up
Outside bullion, the grain has outlived both pounds in a few places, usually because a whole industry standardised on it before metric units arrived.
Aspirin
The standard adult aspirin tablet was once a 5 grain dose. Five grains is 323.99 mg, and the tablet is labelled 325 mg, a figure that comes from rounding the grain to 65 mg. Some packs still print "5 grain" next to the milligrams.
Ammunition
In the US, bullet weights and powder charges are given in grains, so a reloading manual lists a bullet as 55 gr or 150 gr. A 150 grain bullet is 9.72 grams.
Jewellery
Jewellers still weigh scrap gold in pennyweights of 24 grains, twenty to the troy ounce, which is why scrap buyers often quote a price per dwt. One pennyweight is 1.555 grams.
Convert ounces, troy ounces and grams
Twelve units at once, from grains to long tons, with stone and pounds read as written.
The Carat Is Metric, the Karat Is Not a Weight
Gemstones are weighed in carats, and for centuries the carat varied from one trading city to the next. The General Conference on Weights and Measures fixed the metric carat at exactly 200 milligrams in 1907, and the United States adopted it in 1913. A one carat diamond is 0.2 grams, or about 3.09 grains.
The karat of gold jewellery shares the word's origin but measures something else entirely: purity, in 24ths. 24 karat is pure gold, and an 18 karat ring is 18 parts gold to 6 parts other metal, or 75%. No weight converter can turn karats into grams, because they are not a weight.
What a Pound Is Defined As Today
Every unit in this article now hangs off the kilogram. Since 1959 the avoirdupois pound has been exactly 0.45359237 kg, and the grain is defined as 1/7,000 of that pound, which is where 64.79891 mg comes from. The troy ounce follows as 480 grains.
The kilogram itself changed footing on 20 May 2019. Until then it was the mass of a platinum-iridium cylinder kept near Paris. Now it is defined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Nothing about the size of a pound or a troy ounce moved when that happened, because the new definition was chosen to match the old kilogram, but none of them depends on a physical object any more.
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