Troy Weights and the Apothecaries' System

The standard British system of weights is known as the avoirdupois system. It was first used in England around 1300, and its use as a common standard was established by Henry VIII and Elizabeth I. The word 'avoirdupois' is derived from a Norman French phrase meaning 'goods of weight'.

For measuring precious metals and gems, we use the troy system. This is older than avoirdupois; it's probably named after the French market town of Troyes, where English merchants traded at least as long ago as the early 9th century (even before the days of King Alfred). Troyes is in central northern France, about 100 miles east–south–east of Paris. An alternative theory is that 'troy' is a dialect word for a balance.

The apothecaries' system is a variation of the troy system. Apothecary is an archaic term for what we would now call a pharmacist.

Unless otherwise stated, everything that you read below with regard to the troy system also applies to the apothecaries' system.

The smallest unit in all three systems (troy, apothecaries', and avoirdupois) is the grain, and this is also the only unit that's the same in all three systems. It's based on the weight of a grain of barley (a grain of wheat is considered to be three–quarters of a grain), and it's equivalent to approximately one fifteenth of a gram. It was defined in 1959 to be a smidgen under 64.8 milligrams.

The most basic difference between the avoirdupois and troy systems seems to be that in the troy system the units were built up from the smallest – the grain – while in avoirupois the pound was defined first and it was divided into ounces (and drams).

In the troy system: 24 grains make a pennyweight, 20 pennyweights make an ounce, and 12 ounces make a pound.

In the avoirdupois system, a pound is defined as 7,000 grains. The pound is divided into 16 ounces, and the ounce is divided into 16 drams.

This means that both the pound and the ounce, while they're used in both the troy and avoirdupois systems, are different in each.

In the troy system, there are 480 grains in an ounce and 5,760 grains in a pound. In avoirdupois, there are 437.5 grains in an ounce and (by definition, as we've seen) 7,000 in a pound. So the troy ounce is about 9% more than the avoirdupois ounce, but the troy pound is about 18% less than the avoirdupois pound.

A troy pound is 373.24 grams; an avoirdupois pound is 453.59 grams (both figures approximate). The troy pound was however abolished in the UK in the 19th century; troy weights are always given in ounces.

The main difference between the troy and apothecaries' systems is in the subdivision of the ounce. Instead of the pennyweights used in the troy system (as described above), the apothecaries' system has scruples and drachms. A scruple is 20 grains, and a drachm is 3 scruples. Thus (in the apothecaries' system) there are 8 drachms in an ounce and 3 scruples in a drachm.

The word 'dram' (used in the avoirdupois system, as we saw above, for the unit that subdivides the ounce) is obviously derived from the same root as drachm, and the two words are pretty much interchangeable; but convention seems to dictate that the avoirdupois system has drams while the apothecaries' system has drachms. This is particularly pertinent for the quizzer: if you're asked how many drams there are in an ounce, the answer is invariably 16; if you were asked how many drachms there were in an ounce, the answer might well be 8. Note however that 'drachm' is usually pronounced 'dram'; if you're being asked the question verbally, it may be safe to assume that it's about the avoirdupois dram and the answer is 16 – but as always, it might be best to check!

Troy Weights

Grains in a pennyweight		24
Pennyweights in an ounce		20

The Apothecaries' System

Grains in a scruple		20
Scruples in a drachm		3
Drachms in an ounce		8

Grains in a drachm		60
Scruples in an ounce		24

Both Systems

Grammes in a grain (same as in avoirdupois)		0.064 798 91
Grains in a gramme (also the same as in avoirdupois, obviously)		15.432 36
Grains in an ounce (different from avoirdupois, which has 437.5)		480
Ounces in a pound (also different from avoirdupois, which has 16)		12
Grains in a pound (avoirdupois has 7,000)		5,760

© Haydn Thompson 2017