Troy Ounce vs Avoirdupois Ounce
Why gold, silver, and platinum use troy ounces, and how confusing them with grocery ounces costs traders money.
What Is an Avoirdupois Ounce?
The avoirdupois ounce (oz) is the standard unit of mass used for packaged foods, postal parcels, cooking recipes, and everyday items in the United States and the United Kingdom. Under the 1959 International Yard and Pound Agreement codified by NIST SP 811, one international avoirdupois pound contains exactly 16 ounces, fixing the ounce at:
1 avoirdupois ounce (oz) = 453.59237 g ÷ 16 = 28.349523125 grams (exactly 437.5 grains)What Is a Troy Ounce?
A troy ounce (symbol: ozt) is a specialized unit of mass used exclusively in modern commerce for precious metals such as gold bullion, silver ingots, platinum coins, and palladium. Named after the medieval French trade center of Troyes, the troy ounce is based on 480 troy grains:
1 troy ounce (ozt) = 31.1034768 grams (exactly 480 grains)Because 31.1035 g ÷ 28.3495 g ≈ 1.09714, a troy ounce is 9.714% heavier than an avoirdupois ounce.
Side-by-Side Metrological Comparison
| Property | Avoirdupois Ounce (oz) | Troy Ounce (ozt) |
|---|---|---|
| Exact Mass in Grams | 28.349523125 g | 31.1034768 g |
| Mass in Grains | 437.5 grains | 480 grains |
| Ounces per Pound | 16 oz in 1 avoirdupois pound | 12 ozt in 1 troy pound |
| Pound Mass in Grams | 453.59237 g (Heavier pound!) | 373.24172 g (Lighter pound!) |
| Governing Standard | NIST Handbook 44 / BIPM | LBMA / COMEX Bullion Standards |
| Standard Applications | Groceries, mail packages, body weight | Gold bars, silver rounds, platinum bullion |
The Famous Weight Paradox: Gold vs. Feathers
This distinction leads to a classic riddle in physics and metrology: Which is heavier: an ounce of gold or an ounce of feathers? Which is heavier: a pound of gold or a pound of feathers?
Gold wins! Gold is weighed in troy ounces (31.10 g), while feathers are weighed in avoirdupois ounces (28.35 g). One ounce of gold is heavier by 2.75 grams.
Feathers win! Feathers are weighed in avoirdupois pounds (16 oz = 453.59 g), while a troy pound of gold has only 12 troy ounces (12 × 31.10 g = 373.24 g). A pound of feathers is 80.35 grams heavier!
The High Cost of Kitchen Scale Errors
⚠️ Financial Loss Warning for Gold & Silver Sellers
Spot gold prices are quoted per troy ounce (e.g., $2,500/ozt). If you place a 100-gram gold bar on a cheap kitchen scale set to "oz", it will read 3.527 oz (avoirdupois). But in the precious metals market, 100 grams is actually only 3.215 ozt.
Conversely, if you sell an American Gold Eagle coin (1.00 troy ounce / 31.1 g) to a buyer who uses a standard scale reading 1.00 avoirdupois ounce (28.35 g), you are giving away 2.75 grams of pure gold (~$220 in value) for free!
Conversion Formulas Between Troy & Avoirdupois
grams = troy oz × 31.1034768troy oz = grams ÷ 31.1034768troy oz = avoirdupois oz × 0.91145833avoirdupois oz = troy oz × 1.09714286Related Unit Converters
Frequently Asked Questions: Troy vs Avoirdupois
Find answers to common questions about weight units, conversion formulas, and calculations.
How much heavier is a troy ounce than a regular ounce?
A troy ounce is exactly 2.75395 grams heavier than a standard avoirdupois ounce. That makes a troy ounce approximately 9.714% heavier (31.1035 g vs 28.3495 g).
Why do gold and silver use troy ounces instead of regular ounces?
The troy weight system originated in Troyes, France during the Middle Ages as an international trading currency standard for precious metals and gemstones. It survived centuries of legal reforms because financial markets, mints, and bullion dealers required unbroken historical continuity in the purity and valuation of gold and silver coins.
How many troy ounces are in a pound of gold?
A troy pound contains exactly 12 troy ounces (373.242 grams / 5,760 grains), whereas a standard avoirdupois pound contains 16 regular ounces (453.592 grams / 7,000 grains). This means a troy pound of gold is actually lighter than a regular pound of feathers, even though a single troy ounce is heavier than a single regular ounce.
How do I convert avoirdupois ounces to troy ounces?
To convert regular avoirdupois ounces to troy ounces, multiply by 0.911458 (e.g., 10 regular oz = 9.115 troy oz). To convert troy ounces to regular ounces, multiply by 1.09714 (e.g., 10 troy oz = 10.971 regular oz).
Can I weigh scrap gold on a standard kitchen digital scale?
You can, but kitchen scales display regular ounces (28.35 g) by default unless they have a specialized "ozt" mode. If your scale reads 1.00 oz on a kitchen setting, it represents only 28.35 grams—roughly 0.91 troy ounces. Always switch your scale mode to grams (g) or troy ounces (ozt) to avoid severe financial loss.