Two different ounces, and only one of them prices metal
The word "ounce" names two different units, and precious metals use only one of them. A troy ounce is 31.1034768 grams by definition. The ounce on a kitchen scale, a postal scale or a nutrition label — the avoirdupois ounce — is 28.349523125 grams. The troy ounce is about 9.7% heavier, and gold, silver, platinum and palladium have been priced in troy ounces for centuries, without exception.
The gap matters because it runs in one direction only when the mistake is made the common way: weighing metal on a kitchen scale, which reads in avoirdupois ounces, and then pricing that reading against a quote given per troy ounce. Doing that overstates the value by roughly a tenth. Going the other way — weighing in troy and pricing in avoirdupois — understates it by about the same margin. Either error is avoidable by never using ounces from a kitchen scale for precious metal at all: weigh in grams, which is unambiguous, and convert from there.
Where the troy ounce comes from
Troy weight predates the metric system by centuries — the name is usually traced to the French market town of Troyes, a major medieval trading centre — and it survived specifically in bullion and precious-metal trade after the rest of commerce moved to metric or avoirdupois measures. That is why it can feel like an odd holdout: it is one, and it has stayed because switching a global pricing convention has a cost nobody has wanted to pay.
The troy system also has its own subdivisions, most of which show up somewhere in the jewellery trade even today. A troy ounce divides into 20 pennyweight (dwt), still the unit a large share of American scrap-gold buyers weigh in. A pennyweight divides into 24 troy grains, and the troy grain — 1/480 of a troy ounce — turns up on antique balance scales and in old coin specifications, because it was the finest unit the system offered.
The regional units built on the troy ounce
Outside the pennyweight and grain, two more units are fixed multiples of the troy ounce and are worth knowing if gold ever gets bought, sold or gifted across borders. The tola — used across India, Pakistan, Bangladesh (where it is also called a vori or bhori) and much of the Gulf — is exactly three eighths of a troy ounce, 11.6638038 grams. It was standardised at 180 troy grains, which is where the exact fraction comes from.
Thailand prices gold in baht, and uniquely there are two baht weights in active use: the bullion baht at 15.244 grams, which is the figure the Thai Gold Traders Association publishes its daily price against, and the slightly lighter jewellery baht at 15.16 grams. The two are about half a percent apart, which sounds trivial until it is applied to a kilogram bar — treat them as different units, not as the same number with different names.