A pennyweight (abbreviated dwt) is a unit of mass that is equal to 24 grains, 1⁄20 of a troy ounce, 1⁄240 of a troy pound, approximately 0.054857 avoirdupois ounce  and exactly 1.55517384 grammes.
In the Middle Ages, a British penny's weight was literally, as well as monetarily, 1⁄20 of an ounce and 1⁄240 of a pound of sterling silver. At that time, the pound in use was the Tower pound. The medieval English pennyweight was thus equal to 32 Tower grains (also known as wheat 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 5400 Troy grains (also known as barleycorns), the Tower pennyweight 221⁄2 Troy grains (and thus approximately 1.46 grams). After 1527, the English pennyweight was the Troy pennyweight.
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. The Troy ounce enjoys a specific legal exemption from metrication in the UK.
The pennyweight is the common weight used in the valuation and measurement of precious metals. Jewelers 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 crowns and inlays.
The most common abbreviation for pennyweight is dwt; d, for the Roman denarius, was the abbreviation for penny before decimalization of the British monetary system. Alternate abbreviations are pwt and PW.
Although the abbreviations are the same, the pennyweight bears no relation to the actual weight of the penny nail, but to the price for a hundred nails in 15th century England: the larger the nail, the higher the cost per hundred.
- Calculated using the relationships:
- 1 avoirdupois ounce = 437.5 grains
- 1 troy ounce = 480 grains
- 1 avoirdupois ounce = 437.5 grains
- Calculated using the relationship:
- 1 troy grain = 0.06479891 gramme (exact by definition)
- Zupko, Ronald Edward (1985). A dictionary of weights and measures for the British Isles. American Philosophical Society. pp. 280–84. ISBN 978-0-87169-168-2.
- "Weights and Measures Act 1985". August 30, 2011. Retrieved September 22, 2011. "Part II Weighing and Measuring for Trade ... 8 Units of measurement, weights and measures lawful for use for trade ... (2) No person shall use for trade—(a) the ounce troy, except for the purposes of transactions in, or in articles made from, gold, silver or other precious metals, including transactions in gold or silver thread, lace or fringe..."
- "Precious Metal Buyers, Gold Refiners, Gold Assayers, Waste Management". Precious metals calculator. Maguire & Strickland Refining. September 22, 2011. Retrieved September 22, 2011. "This section is for those in the trade. Those in the trade include jewelers, dentists and dental laboratories who have a store front, State ID numbers and Federal ID numbers ... We do not list a dental estimating factor as there is no standard gold percentage for dental solids"
- "Penny" (subscription required). Oxford English Dictionary (2nd ed.). Retrieved 2010-05-30. "Applied to nails, such adjectives denote the original price (in 15th c.) per hundred; as fivepenny nail, a nail which cost 5d. a hundred, tenpenny nail, a nail costing 10d. a hundred. (These names persisted after the prices fell, as they began to do in some places before 1500, and they were eventually used to designate sizes of nails.)"
- H. Littlehales (1905). Medieval Rec. London City ChurchCited in the Oxford English Dictionary under "Penny" with a quote from 1426-1427.
- "Penny". sizes.com. Retrieved 2010-01-10.
- Norman Scott Brien Gras (1918). The Early English Customs System. Harvard University Press, Cambridge (MA). p. 701. Cited at sizes.com with a quote from 1507.
- "Coin Specifications" (table). About us. United States Mint. September 22. Retrieved September 22, 2011. "The following table gives specifications for The United States Mint legal tender coins presently in circulation"