Weight is volume times density, and everything follows
Sheet and wire are the two shapes where that reduces to a single division. Sheet of a constant thickness has a weight per unit of area; wire of a constant cross-section has a weight per unit of length. Divide the weight you have by either one and you have the stock it makes.
sheet area = weight / (thickness x density)
wire length = weight / (cross-section x density)
A troy ounce of sterling at 10.36 g per cm3 is 3.002 cm3 of metal. Rolled to 0.8 mm it covers 37.5 sq cm, which is 5.82 square inches, a piece a shade over 2-3/8 in square. Drawn to 1 mm round wire it is 3,823 mm long, which is 12.5 ft.
The number that surprises people
Sheet area is inversely proportional to thickness, so the yield from a fixed weight changes fast with the gauge you roll to:
| Thickness | Sq in from one troy ounce of sterling | Grams per sq in |
|---|---|---|
| 0.4 mm | 11.63 | 2.67 |
| 0.6 mm | 7.76 | 4.01 |
| 0.8 mm | 5.82 | 5.35 |
| 1.0 mm | 4.65 | 6.68 |
| 1.5 mm | 3.10 | 10.03 |
Wire is worse, because the cross-section goes with the square of the diameter. Halving the diameter quadruples the length. One troy ounce of sterling gives 12.5 ft at 1 mm, 50 ft at 0.5 mm and 3.1 ft at 2 mm. That is why a spool of fine wire lasts and a coil of heavy wire evaporates.
Density is not a property of the name
This is the input worth being careful with. Sterling is nominally 10.36 g per cm3 and fine silver 10.49, a difference of more than one percent before you have done anything. Karat golds are worse: 14 karat yellow, white and rose all carry the same weight fraction of gold and have noticeably different densities, because the balance is a different mix of silver, copper, zinc, nickel or palladium in each.
The consequence is that a table figure gives you an estimate and your supplier gives you an answer. Better still, if you have a piece of the actual stock, weigh it and measure it — a 100 mm length of 1 mm wire weighing 0.81 g puts the density at 10.3 without anybody having to be trusted.
Three ounces, all different
The unit confusion in this corner of the trade is real and it costs money.
| Unit | Grams | Note |
|---|---|---|
| Grain | 0.0648 | 480 to the troy ounce |
| Pennyweight (dwt) | 1.5552 | 20 to the troy ounce |
| Ounce, avoirdupois | 28.3495 | the kitchen scale ounce |
| Troy ounce (ozt) | 31.1035 | what metal is priced in |
| Pound, avoirdupois | 453.59 | 16 avoirdupois ounces |
| Pound, troy | 373.24 | 12 troy ounces, not 16 |
A troy ounce is about 9.7 percent heavier than an ordinary ounce, and a troy pound is lighter than an ordinary pound despite the ounce being heavier, because there are only twelve of them in it. Every figure on this page carries its unit for that reason.
What the allowance is really for
The offcut percentage on the form is not waste in the ordinary sense. Filings, saw dust, the edge trim and the piece that got cut backwards all go into the bench pan, and the pan goes to a refiner. What you get back depends entirely on the terms your refiner offers and on how much of it is actually metal rather than steel wool, wax and sanding grit. So the allowance is best read as capital tied up in the pan rather than as metal consumed.
How big it should be depends on the work. Rectangles cut from a strip lose almost nothing. Small round shapes nested into a sheet can leave a third of it behind as web, and the only reliable way to know your own figure is to weigh what you buy, weigh what leaves as finished work, and look at the difference over a few months.
Questions people ask
How many square inches is a troy ounce of sterling sheet?
At 0.8 mm thickness and 10.36 g per cm3, 5.82 square inches — a piece about 2-3/8 in square. At 0.4 mm it doubles to 11.63 sq in and at 1.5 mm it drops to 3.10. Area is inversely proportional to thickness, so the gauge you roll to matters more than anything else in the calculation.
How much wire does a troy ounce of silver make?
At 1 mm diameter, 12.5 feet of sterling. Length goes with the inverse square of the diameter, so the same ounce gives 50 feet at 0.5 mm and 3.1 feet at 2 mm. Doubling the wire diameter cuts the length to a quarter, which is why heavy-gauge wire disappears so much faster than it feels like it should.
What is the difference between a troy ounce and a regular ounce?
A troy ounce is 31.1035 g and an avoirdupois ounce is 28.3495 g, so the troy ounce is about 9.7 percent heavier. Precious metal is priced and sold in troy ounces and pennyweight, where a pennyweight is a twentieth of a troy ounce at 1.5552 g. A troy pound is twelve troy ounces, 373.24 g, which is lighter than an ordinary pound.
What density should I use for karat gold?
Whatever your supplier gives for the specific alloy on the invoice. Fourteen karat yellow, white and rose all contain the same weight fraction of gold and have quite different densities, because the rest of the alloy is a different mix in each. A table figure gets you an estimate. If you have the stock in hand, weighing a measured length of it is a better answer than any published number.
How much extra metal should I order for a job?
That depends on the shapes, and the honest answer is to measure your own. Rectangular pieces cut from a strip lose almost nothing; small round parts nested into a sheet can leave a third behind as web. Fifteen percent is a starting figure for straightforward flat work. None of it is truly lost, since filings and offcuts go to a refiner, but it is tied up until they do.