apothecaries' weight

A system of units of mass used by druggists in the English-speaking world, before 15th – 19th century. The pound, ounce, and grain of apothecaries' weight are the same sizes as the corresponding units in troy weight, but the scruple and dram are not found in troy weight. The division of the pound into 12 ounces is undoubtedly modeled on the Roman libra, but how the apothecaries' pound got its present value is controversial.

pound

ounce

12

dram

8

96

scruple

3

24

288

grain

20

60

480

5760

0.0648
grams

1.296
grams

3.888
grams

31.10
grams

0.373
kilograms

By the middle of the 18th century, English druggists were using avoirdupois weight instead of apothecaries' weight, at first to measure what they sold, and then for compounding medicines as well. By the middle of the 19th century apothecaries' weight had largely disappeared. In the United Kingdom the Medical Act of 1858 required the use of avoirdupois weight; the Weights and Measures Act of 1878 retained only the apothecaries' ounce, and that merely permissively.

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