Donaldson R F Troy Ounce, Pennyweight & Grain Converter
The weight system used for gold, silver and gemstones — not the same ounce as cooking or the post office.
How this is worked out
The troy system is a separate weight scale from the everyday (avoirdupois) ounce and pound — a troy ounce (31.1034768 g) is heavier than the familiar avoirdupois ounce (about 28.35 g), which is why a "pound" of gold (12 troy ounces) isn't the same as a 16-ounce avoirdupois pound of anything else. The grain is the one unit shared by both systems, fixed at exactly 64.79891 mg since the international yard-and-pound agreement of 1 July 1959. From there, 24 grains make a pennyweight and 20 pennyweights (480 grains) make a troy ounce — both exact, not rounded.
Frequently asked questions
Where is pennyweight still used?
Mainly in jewellery-making and bullion trading in English-speaking countries — it's a convenient middle-sized unit, smaller than a troy ounce but larger than a grain, well suited to weighing individual rings or small pieces.
Is a "troy pound" the same as a normal pound?
No — a troy pound is 12 troy ounces (373.24 g), while an avoirdupois pound (the everyday one) is 16 avoirdupois ounces (453.59 g); the troy pound is actually the lighter of the two, despite the troy ounce itself being heavier than an avoirdupois ounce.
Related tools: Gold Purity & Alloy Calculator, Unit Converter, Molar Mass Calculator.
Source / Last verified: the grain is defined as exactly 64.79891 mg by the international yard-and-pound agreement of 1 July 1959 (which fixes the avoirdupois pound at exactly 0.45359237 kg = 7,000 grains); the troy ounce/pennyweight/grain relationships (24 grains = 1 dwt, 480 grains = 20 dwt = 1 troy oz) are from NIST Handbook 44, Appendix C, "General Tables of Units of Measurement", also reproduced in NIST's own "Precious Metals Conversion Information" reference sheet — read 1 October 2026.