What a pennyweight is
A pennyweight is a unit of troy weight, written dwt. It is a fixed definition: one pennyweight is 24 grains, and 20 pennyweights make one troy ounce.
Because the grain is itself defined from the pound, the pennyweight comes to an exact figure in grams. NIST Handbook 44, Appendix C, gives the grain as 64.798 91 milligrams exactly. Multiplying by 24 gives 1.555 173 84 grams, which is the figure this converter uses. The same chain, grain to pennyweight to troy ounce, appears in the UK Weights and Measures Act 1985, Schedule 1. It defines the grain as 1/7000 of a pound of 0.453 592 37 kg and the pennyweight as 24 grains, and its Part V gives the troy ounce as 0.031 103 476 8 kg. Twenty times 1.555 173 84 g is 31.103 476 8 g, so the definitions agree to the last digit.
Why jewellers still use it
In the United States the pennyweight is still a common unit for weighing scrap gold, castings and findings at the bench. It is a convenient size: a plain wedding band might weigh a few pennyweights, where in troy ounces it would be an awkward fraction. Weights, and sometimes prices, are quoted per pennyweight in parts of the trade for that reason.
Elsewhere, and in most retail settings, grams have replaced it. In the UK the Weights and Measures Act lists the pennyweight in Part VI of Schedule 1, the part for units that may not be used for trade except as supplementary indications. A UK trader weighing gold for sale works in grams, and may show pennyweights only alongside.
The practical result is that the same piece can be described in two units depending on who weighed it. A receipt in dwt and a scale reading in grams describe the same metal, and this converter makes the two comparable.
Pennyweight, troy ounce and the ounce on a kitchen scale
The troy ounce, 20 pennyweights or 480 grains, is the ounce in which gold, silver, platinum and palladium are priced. It is not the ounce a kitchen or postal scale shows. That is the avoirdupois ounce, 1/16 of a pound, which is 437.5 grains or 28.349 523 125 g. A troy ounce is about 9.7% heavier.
Confusing the two is the commonest weighing mistake with precious metal. A piece weighed in avoirdupois ounces and priced per troy ounce is understated by close to a tenth. The troy ounce converter on this site sets the two ounces side by side. This page stays with the troy family: grains, pennyweights and troy ounces, plus grams.
Worked examples
A ring weighing 3.2 dwt is 3.2 × 1.555 173 84 = 4.977 g. A chain weighing 15 g is 15 ÷ 1.555 173 84 = 9.645 dwt. Ten pennyweights are 15.552 g, and twenty are exactly one troy ounce, 31.103 g.
These are weights of the whole object. A 14K chain weighing 10 dwt does not contain 10 dwt of gold. 14K is 585 parts per thousand gold, so it contains 5.85 dwt of gold, and the other 4.15 dwt is alloying metal. The scrap gold calculator applies the purity, and the live price, to a weight entered in pennyweights.
Weighing in pennyweights accurately
Many jewellery scales can switch between grams, pennyweights and troy ounces, and the abbreviation on the display matters. “dwt” is pennyweight. “ozt” or “oz t” is troy ounce. A bare “oz” is usually avoirdupois. “gr” on some scales means grains, while “g” means grams.
Because the conversion is exact, nothing is lost by weighing in grams and converting here, or the other way round. What matters is the scale’s own resolution and calibration, and that the piece is weighed on its own, without packaging or a bag.