What Is a Troy Ounce?

Troy ounce vs ordinary ounce

One troy ounce equals about 1.0971 avoirdupois ounces. The pound works the other way round: a troy pound has 12 troy ounces (373.24 g) while an avoirdupois pound has 16 ordinary ounces (453.59 g), so a troy pound is lighter. Precious metals today are quoted almost only in troy ounces, grams and kilograms; troy pounds are rarely used.

Where the troy ounce comes from

The name is usually traced to Troyes, a market town in Champagne, France, whose medieval fairs drew merchants from across Europe. The troy system was adopted in England for coinage and bullion, and its base unit, the grain, links it to the avoirdupois system: both use the same grain of 64.79891 milligrams. In 1959 the English-speaking countries agreed exact metric definitions for the yard and pound, which fixed the troy ounce at exactly 31.1034768 grams. That definition is what the London bullion market, COMEX and price feeds use today.

Conversion table

For any other unit, including vori, tael and baht, use the gold unit converter.

Converting an ounce price into a gram price

Divide the price per troy ounce by 31.1035. Using an illustrative spot price of US$4,150 per ounce:

• Per gram: $4,150 ÷ 31.1035 = $133.43
• Per kilogram: $4,150 × 32.1507 = $133,426
• Per tola: $4,150 × 0.375 = $1,556.25
• Per gram of 22K: $133.43 × 0.916 = $122.22

The live versions of these figures are on the gold price per gram, per kilogram and per tola pages.

Common confusions

Dividing by 28.35 instead of 31.10

This is the most frequent mistake in home calculations and some apps. Dividing $4,150 by 28.3495 gives $146.39 per gram, about 9.7% too high. If a gram price looks oddly expensive against what dealers quote, check which ounce was used.

Weighing coins on a kitchen scale

A kitchen or postal scale set to ounces reads avoirdupois ounces. A genuine 1-oz gold coin weighs 31.1 g and will show as about 1.10 oz on such a scale, which sometimes makes owners think it is overweight. Switch the scale to grams.

Gross weight vs fine weight

Many bullion coins are not pure. A 22-karat coin sold as "1 ounce" contains one troy ounce of pure gold but weighs more, about 33.9 g, because of the added copper or silver. The price follows the fine gold content, not the total weight. See gold coin prices.

"Ounce" in jewellery shops

Retail jewellery is priced by the gram almost everywhere, including the US. The troy ounce appears mainly for bullion bars, coins and market quotes, so convert before comparing a jewellery price with spot. Take an 18K ring weighing 6 g priced at $1,200: that is $200 per gram of metal, or $200 ÷ 0.75 = $266.67 per gram of pure gold. Against an illustrative spot of $133.43 per gram, the gold accounts for about half the price and the rest is design, labour, retail margin and any stones.

Pennyweights in the US jewellery and scrap trade

American jewellers, refiners and scrap buyers often weigh in pennyweights (dwt), another troy unit: 20 dwt make one troy ounce, so 1 dwt = 1.5552 g. Suppose you sell 5 dwt of 14K scrap. That is 5 × 1.5552 = 7.776 g of metal, containing 7.776 × 0.585 = 4.549 g of pure gold. At an illustrative $133.43 per gram, the melt value is 4.549 × $133.43 = $606.97, and a buyer will offer a percentage of that. The scrap gold calculator does this conversion for grams or pennyweights.

Standard bar sizes in troy ounces

Retail bullion is sold in both systems. Common sizes include 1 oz, 10 oz and 100 oz bars alongside metric bars of 1 g to 1 kg. The COMEX futures contract represents 100 troy ounces, and the London Good Delivery bar is about 400 troy ounces, roughly 12.4 kg. A 1-kilogram bar contains 32.15 troy ounces, which is why kilo bars are priced a little above 32 times the ounce price.