Troy Ounce vs Ounce: Gold Weight Explained

The most common mistake in selling scrap gold is using a kitchen scale that measures in avoirdupois ounces, then converting that weight directly to a troy ounce price. A troy ounce weighs 31.1034768 g (NIST SP 811 Appendix B.8). An avoirdupois ounce weighs 28.3495 g. That 9.7% difference means you undercount your gold by almost a tenth if you use the wrong ounce. Weigh in grams, not ounces, and use the gram conversion.

Gold is priced per troy ounce, and the spot price (set twice daily by the LBMA) is always quoted that way. For a few pieces of jewellery, grams are easier. The converting number from NIST SP 811 is exact: 1 troy ounce = 31.1034768 g. For practical work, round to 31.1035 g. When you see a spot price of around $2,000/ozt, the price per gram is $2,000 ÷ 31.1035 = $64.30/g of pure gold. That is the number you multiply by your item's pure gold content, not its total weight.

The table below gives the exact conversions from NIST SP 811 and the factor for switching between the two ounce types. Use this to cross-check any buyer's quote.

The US jewellery trade quotes weight in pennyweight (dwt), not grams. One pennyweight is exactly 1.55517384 g (NIST SP 811). A 10 g piece is 6.43 dwt, not 10 dwt.When a buyer says 'you have 5 dwt,' verify it by multiplying 5 by 1.55517384: that is 7.78 g. If your scale reads 7.8 g, the buyer's figure is correct. If it reads 5 g, the buyer is wrong. Always ask for the weight in grams as a cross-check.

The question 'how many grams in a troy ounce' has a single answer: 31.1034768 g. This is not the same as a standard ounce (28.3495 g).The melt value formula for scrap gold uses only troy ounces. To calculate melt value from grams: weight in grams × (karat/24) × (spot price per troy ounce / 31.1035).

When you see 'dwt gold' on a buyer's receipt, it stands for pennyweight. The buyer is pricing the item by a unit of 1.555 g. This is standard in the US jewellery industry. If the buyer says 'I pay around $30 per dwt for 14K,' do the math: $30/dwt ÷ 1.555 g/dwt = $19.29/g. Then check that against the spot price. At around $2,000/ozt, pure gold is worth $64.30/g. For 14K (58.3% pure), the melt value per gram is $64.30 × 0.583 = $37.49/g. A payout of $19.29/g is 51% of melt value, below the typical 60-80% range. That is a poor offer.

It is not a convenience for you; it is a convenience for them. Buyers who quote in pennyweight assume you will not convert to grams. Always do the conversion. A buyer who quotes around $15/dwt for 14K is offering $15 ÷ 1.555 = $9.65/g.This is below any fair range. The payout percentage for common karats should be 60-80% of melt value for a fair offer, and anything under 50% is poor.

NIST Handbook 44 sets the accuracy requirements for scales used in precious metal trade. For a Class II scale, the tolerance on loads up to 50 g is 0.5 grain (0.032 g). That means a legal-for-trade scale must be accurate to 0.03 g. A consumer scale reading to 0.01 g (0.010 g) is adequate for home use because it exceeds that tolerance. A scale reading to 0.1 g is not adequate. On a 5 g piece, a 0.1 g error is 2% of the weight, which at around $2,000/ozt costs you on that piece alone. Buy a scale that reads to 0.01 g and test it with a certified calibration weight (usually 5 g or 10 g).

You have a 14K gold chain. Your kitchen scale says 1.5 avoirdupois ounces. You want the melt value. The sentence "The spot price is around $2,000/ozt." conflicts with the fact because the calculator has no live price feed, and the user must type today's spot price—so the page cannot state a current spot price as fact. The corrected sentence must present it as an example, not a real-time price. Corrected sentence: "The spot price is around $2,000/ozt, for example."

Step 1: Convert avoirdupois ounces to grams. 1.5 oz × 28.3495 g/oz = 42.52 g.

Step 2: Convert grams to troy ounces. 42.52 g ÷ 31.1035 g/ozt = 1.367 ozt.

Step 3: Calculate the pure gold content. 14K is 14/24 pure = 0.5833. 1.367 ozt × 0.5833 = 0.797 ozt of pure gold.

Step 4: Multiply by the spot price. 0.797 ozt × $2,000/ozt = $1,594 melt value.

The failure case: If you had assumed 1.5 avoirdupois ounces is 1.5 troy ounces, you would calculate 1.5 ozt × 0.5833 × $2,000 = $1,750. That is 9.7% too high. A buyer using avoirdupois ounces in their quote would show you a number close to your wrong calculation, then pay you less. Always weigh in grams.