AboutMyCoinWhat is a troy ounce?
A troy ounce is the unit precious metal is weighed in, and it is 31.1035 grams — about a tenth heavier than the 28.3495 gram ounce used for everything else. Metal weights and spot prices are quoted per troy ounce, because mixing the two units overstates what you have by a tenth.
The two ounces compared
One troy ounce is 31.1035 grams. One avoirdupois ounce, the one on a kitchen scale and on a packet of flour, is 28.3495 grams. The troy ounce is the heavier by about 10%, and the two share a name because they descend from different medieval systems that both survived.
The confusion runs the other way as well. A troy pound is twelve troy ounces rather than sixteen, so a troy pound is lighter than a regular pound even though a troy ounce is heavier than a regular ounce. Precious metal is never quoted in pounds, which is how the trade avoids the problem entirely.
Why it matters on a coin
Spot prices are quoted per troy ounce, so a metal weight has to be in troy ounces before the multiplication means anything. Weigh a coin on a kitchen scale, read grams, and divide by 31.1035 — not by 28.3495, which inflates the answer by a tenth before you have started.
Worked through: a 1964 Washington quarter weighs 6.25 grams and is 90% silver, so it holds 5.625 grams of silver, which is 0.1808 troy ounces. That figure times the spot price of silver is the coin’s melt value, and it is how every figure in the melt value section is produced.
Fineness is the other half
A coin’s weight is not its metal content, because almost no circulating coin was struck in pure metal — silver coinage was alloyed with copper to survive being carried. United States 90% silver coinage is nine parts silver to one part copper, and it is the nine parts that the price applies to.
So the calculation always has two steps: gross weight times fineness gives the metal content, and metal content times spot gives the value. Getting the fineness right matters as much as getting the unit right, which is why every coin page here states the weight and the composition separately.
A shorthand worth memorising
One dollar of face value in United States 90% silver dimes, quarters or half dollars contains about 0.715 troy ounces of silver once circulation wear is allowed for. Four quarters, ten dimes or two halves — the mix does not matter, because the denominations were struck to the same weight per dollar.
That single number prices a jar of junk silver in one multiplication, which is why the trade uses it in preference to counting coins.
Related questions
More on silver, gold and spot prices
The metal in a coin is the part of its value that is calculated rather than judged.
Silver, Gold and Spot Prices collects every question on this topic in one place.
Not an appraisal. AboutMyCoin estimates from published specifications and does not authenticate or grade coins.