The numbers
- 1 pennyweight (dwt) = 24 grains = 1.555 g (1.555 173 84 g exactly)
- 20 pennyweights = 1 troy ounce = 31.103 g
- 1 gram = 0.643 pennyweight
Converting
Multiply pennyweights by 1.555 to get grams. A 10 dwt casting weighs 10 × 1.555 = 15.55 g, which is half a troy ounce.
To go the other way, multiply grams by 0.643. NIST’s own example: a 4.40 g piece of jewellery is 4.40 × 0.643 = 2.83 dwt.
Pricing by the pennyweight
Gold prices are quoted per troy ounce. Divide by 20 for the price per pennyweight. A gold buyer may then pay less than that, depending on the karat of the piece.