Weight is volume times density
The weight of a solid piece of metal is its volume multiplied by the metal’s density. The volume comes from the shape. A block or sheet is length × width × thickness. A rod or wire is π × radius² × length. A ball is 4/3 × π × radius³. These formulas are exact geometry, and inches convert to millimetres exactly, at 25.4 mm to the inch.
The density is the published figure for the pure element at room temperature, from the CRC Handbook of Chemistry and Physics, the same figures used in the metal density comparison on this site. Pure gold is 19.32 g/cm³, fine silver 10.49, platinum 21.45 and palladium 12.02.
How much a cubic inch of gold weighs
A cubic inch is exactly 16.387 064 cm³, a figure NIST Handbook 44 lists as exact. Multiplied by 19.32 g/cm³, a solid cubic inch of pure gold weighs about 316.6 g, a little over ten troy ounces (10.18 ozt). The same cube in fine silver weighs about 171.9 g, and in platinum about 351.5 g.
A cubic centimetre is simpler: it weighs the density itself. A 10 mm cube is one cubic centimetre, so a 10 mm cube of pure gold weighs 19.32 g and of fine silver 10.49 g.
Gold’s density is why a valuable quantity of it is so compact. A London Good Delivery gold bar holds between 350 and 430 troy ounces of fine gold under the LBMA specification; 400 troy ounces of pure gold occupy about 644 cm³.
Why only pure metals
Karat gold, sterling silver and the platinum alloys used in jewellery are left out, deliberately. Their density depends on which metals the maker added and in what proportions. 18K gold is 75% gold by weight, but the other 25% might be mostly silver, mostly copper, or nickel or palladium for white gold, and each gives a different density. There is no single figure for “18K gold”, and this site does not publish a guessed one.
For the same reason, the weight of an 18K shape is not 75% of the pure-gold result. The alloying metals are less dense than gold, so the alloy takes up more space per gram than gold does. Applying the karat fraction to the pure-gold weight answers a different question and gives the wrong number.
Brass and stainless steel are left out too. The metal density comparison stores them as ranges, because their composition varies, and a weight calculated from a midpoint would carry false precision.
Running it backwards
The same relationship can be used in reverse. If a bar is sold as pure metal, its weight can be compared with what its dimensions predict. A solid fine silver bar measuring 50 × 25 × 10 mm should weigh about 131 g. A bar of that size weighing much less is not solid fine silver.
Dimensions alone cannot prove a piece genuine, however. Tungsten’s density, 19.25 g/cm³, is within 0.4% of gold’s, which is why tungsten-cored fakes can pass a size-and-weight check. The specific gravity calculator measures density directly by weighing in air and in water, and only an assay establishes composition.
What the result does not include
The result is the weight of an ideal solid shape. Real objects have rounded edges, hollows, engraving, holes and wear, all of which remove metal, and castings can contain internal porosity. Measuring a real piece with calipers and treating it as a perfect block gives an upper bound, not its weight. For a piece in hand, weighing it is always more accurate than calculating it.
No prices appear on this page. To put a value on a weight of pure metal, the metal value calculators apply the live spot price to a weight you enter, and they show melt value only, not what a buyer would pay.