The Pennyweight

Although hardly used in the bullion market, the pennyweight is still in use in dentistry and jewellery in the UK. A pennyweight (dwt) is now defined as a unit of mass equal to 24 grains, 1⁄20 of a troy ounce and exactly 1.55517384 grams. It is abbreviated as dwt, with the ‘d’ standing for denarius, an ancient Roman coin, later used as the symbol of the pre-decimalisation British penny.

Originally, the penny was both a weight and a monetary unit. The first ‘modern’ pennies were introduced by King Offa of Mercia around 775. His first series were equal to 1⁄240 of a Saxon pound of silver (the latter being defined as 5,400 grains, or approximately 350 grams), giving a pennyweight of about 1.46 grams. Offa’s coins were imitated by East Anglia, Kent, Wessex and Northumbria, as well as by two Archbishops of Canterbury.

In the Middle Ages, an English penny’s weight was literally, as well as monetarily, 1⁄20 of an ounce and 1⁄240 of a pound of sterling silver. The purity of sterling silver was set at 92.5% in 1158 by Henry II. At that time, the pound unit in use in England was the Tower pound, equal to 7,680 Tower grains (also known as wheat grains). The medieval English pennyweight was thus equal to 32 Tower grains. When troy weights replaced Tower weights in 1527, the troy weights were defined in such a way that the old Tower pound came out to exactly 5,400 troy grains (also known as barleycorns) and the Tower pennyweight to 22½ troy grains (and thus approximately 1.46 grams). After 1527, the English pennyweight was the troy pennyweight of 24 troy grains. Thus, the troy pound, ounce and pennyweight, with their definitions given in terms of the troy grain instead of in terms of the Tower grain, were 1⁄15 or 6.667% more than the Tower equivalents.

The troy pound and the pennyweight lost their official status in the United Kingdom in the Weights and Measures Act of 1878; only the troy ounce and its decimal subdivisions remained official (and only for use in the precious metals market). The European Union granted the troy ounce a specific legal exemption from metrication in the UK.

Mining companies used to describe the gold content of ores in terms of dwt/ton before the use of g/tonne became ubiquitous. Jewellers use the pennyweight in calculating the amount and cost of precious metals used in fabricating or casting jewellery. Similarly, dentists and dental labs still use the pennyweight as the measure of precious metals in dental restorations.