Troy ounce, gram, kilo: gold weight units explained

Gold prices are quoted in troy ounces. This causes confusion because the word "ounce" in everyday use means something different: the avoirdupois ounce used for food, body weight, and most common measurement. The two systems are not compatible, and conflating them produces errors in any calculation that touches real gold. For a working code example of these conversions, see Convert gold prices across currencies in code.

The avoirdupois ounce vs the troy ounce

The avoirdupois system is what most people mean when they say "ounce." It's the system used in the United States for everyday weights.

1 avoirdupois pound = 16 avoirdupois ounces
1 avoirdupois ounce = 28.3495 grams

The troy system is older and was historically used for precious metals and gemstones. It survives today almost exclusively in gold, silver, platinum, and palladium pricing.

1 troy pound = 12 troy ounces
1 troy ounce = 31.1035 grams

The troy ounce is heavier than the avoirdupois ounce. Specifically, 1 troy ounce = 1.09714 avoirdupois ounces. That difference is about 10%, which is not trivial in commercial gold transactions.

The troy pound, however, is lighter than the avoirdupois pound, because there are only 12 troy ounces to the pound instead of 16. This is counterintuitive but accurate. The troy pound is not used in practice; gold is always weighed and quoted in troy ounces directly.

Why troy ounces?

The troy weight system traces back to Troyes, a trading city in medieval France that hosted major European trade fairs. The uniformity of weights used there for precious metal transactions became a standard, and it was codified in English law centuries ago. The US adopted the troy ounce for precious metals formally in 1828.

The system persisted not because of inertia alone, but because the global gold market (London in particular) adopted it as its standard and never changed. Today, every spot price, every futures contract, every bank quote, and every gold ETF NAV expresses gold in troy ounces. If you want to participate in the global market, you work in troy ounces.

Converting spot price to grams and kilograms

The spot price is stated as price per troy ounce. To get a price per gram or per kilogram, divide or multiply by the conversion factor.

Per gram:

Price per gram = spot price ÷ 31.1035

If gold is at $2,400 per troy ounce:
$2,400 ÷ 31.1035 = $77.16 per gram

Per kilogram:

Price per kilogram = spot price × 32.1507

The multiplier 32.1507 comes from the number of troy ounces in one kilogram:
1 kilogram = 1,000 grams ÷ 31.1035 grams/toz = 32.1507 troy ounces

If gold is at $2,400 per troy ounce:
$2,400 × 32.1507 = $77,162 per kilogram

A sanity check: a kilogram of gold at $2,400/toz is worth about $77,000. That number is useful for calibrating whether a quoted price or API value is in the expected range for the unit being claimed.

The tola, the baht, and other regional units

Several regional markets trade and quote gold in local weight units. Developers building applications for those markets should know the conversions:

Tola (South Asia, particularly India and Pakistan): 1 tola = 11.6638 grams = 0.375 troy ounces

Baht (Thailand): 1 baht = 15.244 grams = 0.4901 troy ounces

Tael (Hong Kong, China, parts of Southeast Asia): the Hong Kong tael = 37.429 grams = 1.2034 troy ounces. Note: Chinese tael and Taiwanese tael differ slightly; always confirm which is meant.

Grain: 1 troy ounce = 480 grains. Rarely used commercially, but appears in older texts.

Practical implications for API users

When you pull a gold price from an API, the price per troy ounce is the standard format. If your application displays or computes in grams or kilograms, you need to convert explicitly.

The critical thing to check: what unit is the API actually returning? Most well-documented APIs state this clearly in the response schema (e.g., unit: "XAU" where XAU is the ISO 4217 currency code for gold, defined as one troy ounce). If the unit is not documented, assume troy ounce unless the price range makes no sense. A per-gram price in USD at today's prices should be around $70–80, while a per-toz price should be around $2,200–2,500.

Getting the unit wrong propagates through every downstream calculation. A display showing "$2,400 per gram" would imply gold costs $74.8 million per kilogram. The arithmetic always reveals the error, which is why explicit unit handling matters more than most developers initially expect.

Conversion reference

These conversion factors don't change. When in doubt, run the number through them before trusting a price display. You can verify your numbers against the live spot price at any time.