A troy ounce is 31.1034768 grams. The everyday avoirdupois ounce — used for food, body weight, and shipping — is 28.3495 grams. The troy ounce is about 9.7% heavier. All silver, gold, platinum, and palladium spot prices worldwide are quoted per troy ounce.
This distinction matters in practice. At gold above $4,000/ozt, confusing a troy ounce with an avoirdupois ounce creates a $400 error on a single ounce of metal.
The exact value and its source
1 troy ounce = 31.1034768 grams (exact, by definition)
This value comes from NIST Handbook 44, Appendix C: “1 apothecaries or troy ounce = 31.103 476 8 grams.” It is a defined, exact conversion — not a measurement or an approximation. The troy system is codified in NIST Handbook 130 under Section 2.17, which specifies that precious metals sold by weight must use troy weight or SI units (grams).
The troy system traces to Troyes, France, where medieval trade fairs standardized weights for gold and silver exchange. The system was formalized in Britain and the United States and has remained the global precious metals standard since commodity exchanges adopted it in the 19th century.
The three ounces to know
There are three systems that use the word “ounce.” All three matter when handling precious metals:
| System | Ounce definition | Grams | Common use |
|---|---|---|---|
| Troy | 1/12 troy pound | 31.1034768 | Precious metals, coins, bullion |
| Avoirdupois | 1/16 avoirdupois pound | 28.3495 | Groceries, shipping, body weight |
| Apothecary | 1/12 apothecary pound | 31.1034768 | Historical pharmacy (= troy) |
The apothecary ounce equals the troy ounce exactly — the two systems share the same ounce but differ in how they define pounds. For precious metals, only the troy ounce matters.
Full conversion table
| To convert | Multiply by | Result |
|---|---|---|
| Troy oz → grams | × 31.1034768 | grams |
| Grams → troy oz | ÷ 31.1034768 (× 0.032151) | troy oz |
| Troy oz → pennyweight (dwt) | × 20 | dwt |
| Dwt → troy oz | ÷ 20 (× 0.05) | troy oz |
| Troy oz → avoirdupois oz | × 1.09714 | avdp oz |
| Avoirdupois oz → troy oz | × 0.91146 | troy oz |
| Grams → pennyweight | × 0.64301 | dwt |
| Pennyweight → grams | × 1.55517 | g |
| Troy oz → kilograms | × 0.031103 | kg |
| Kilograms → troy oz | × 32.1507 | troy oz |
| Troy oz → grains | × 480 | grains |
| Grains → troy oz | ÷ 480 | troy oz |
| Avoirdupois lb → troy oz | × 14.5833 | troy oz |
| Troy oz → avoirdupois lb | × 0.068571 | avdp lb |
Worked examples
100g silver bar (.999 fine)
Silver content: 100 × 0.999 = 99.9 g fine silver
Troy oz: 99.9 ÷ 31.1034768 = 3.212 ozt
Melt at $60.73/ozt (Oct 7, 2026): 3.212 × $60.73 = $195.06If you mistakenly used avoirdupois ounces:
100g ÷ 28.3495 g/avdp-oz = 3.527 avdp-oz
3.527 × $60.73 = $214.20 ← overstated by about $19A $19 error on a 100g bar. On a 500g silver bar, that error becomes $95.
Morgan dollar (26.73g, 90% silver)
Silver content: 26.73 × 0.900 = 24.057 g Ag
Troy oz: 24.057 ÷ 31.1034768 = 0.7734 ozt
Melt: 0.7734 × $60.73 = $46.95 (≈ $47.01 from data)14K gold ring (5g)
Gold content: 5.00 × (14 ÷ 24) = 5.00 × 0.5833 = 2.917 g Au
Troy oz: 2.917 ÷ 31.1034768 = 0.0938 ozt
Melt: 0.0938 × $4,134.80 = $387.84For a pennyweight calculation: 2.917 g × 0.64301 = 1.876 dwt.
The kitchen scale trap
Kitchen scales display avoirdupois ounces (labeled “oz”). If your scale reads “3.5 oz” for a silver item, that is 3.5 avoirdupois ounces, not 3.5 troy ounces. To get the correct melt value:
3.5 avdp-oz × 28.3495 g/avdp-oz = 99.22 g
99.22 g ÷ 31.1034768 = 3.190 troy oz
3.190 × $60.73/ozt = $193.73If you had used 3.5 as troy oz directly:
3.5 × $60.73 = $212.56 ← $18.83 too highTwo alternatives to manual conversion: set your scale to grams and divide by 31.1035, or look for a scale with an ozt or t oz mode — many precision jewelry scales include this. The Troy Ounce Converter handles any combination of units in either direction.
Pennyweight — the jewelry and scrap gold unit
Pennyweight (dwt) is common in the scrap gold and jewelry trade. Many buyers quote gold prices per pennyweight. The key relationships:
1 troy oz = 20 dwt
1 dwt = 1.5552 g = 0.050 troy ozChecking a per-dwt quote against melt: If a buyer offers $200/dwt for 14K gold, evaluate it:
14K purity: 14 ÷ 24 = 0.5833
Gold per dwt of 14K: 1.5552 g × 0.5833 = 0.9073 g pure Au
Gold per dwt in troy oz: 0.9073 ÷ 31.1035 = 0.02917 ozt
Spot value of 1 dwt of 14K gold: 0.02917 × $4,134.80 = $120.61A $200/dwt offer would be 66% above spot melt — an unusually high quote worth scrutinizing. A $90/dwt offer would be 75% of melt — acceptable but below a good refiner’s rate. Run this calculation for any per-dwt quote before accepting.
Grains — the smallest traditional unit
The grain (gr) is the base unit of the troy system:
- 24 grains = 1 pennyweight
- 480 grains = 1 troy ounce
- 1 grain = 0.0648 grams
Grains appear on older assay reports and in historical coin specifications. The 1792 Coinage Act specified coin weights in grains: the silver dollar was to weigh 416 grains (27.0 g gross). Modern coin spec tables still sometimes list grains alongside grams.
Troy pounds vs avoirdupois pounds
The troy and avoirdupois systems use different pounds:
- 1 troy pound = 12 troy ounces = 373.24 grams
- 1 avoirdupois pound = 16 avoirdupois ounces = 453.59 grams
Precious metals are never priced by the troy pound — always by the troy ounce. If you see a price “per pound” for gold or silver from a retail source, it is almost certainly avoirdupois pounds and requires conversion. Copper is the exception: copper spot prices are quoted per avoirdupois pound (e.g., $6.57/lb = $6.57 per 453.59 grams).
Common unit summary for melt value work
| Unit | Grams | Troy oz | Pennyweight |
|---|---|---|---|
| 1 troy oz | 31.1035 | 1.0000 | 20.000 |
| 1 dwt | 1.5552 | 0.0500 | 1.000 |
| 1 grain | 0.0648 | 0.00208 | 0.0417 |
| 1 avdp oz | 28.3495 | 0.9115 | 18.229 |
| 1 gram | 1.0000 | 0.03215 | 0.6430 |
Keep this table handy when comparing quotes from dealers who use different units. The Troy Ounce Converter converts any of these units to any other in one step.