Troy & Tower Weights Pennyweights and Carats

Troy weights were first used in England in the 1400s.

In 1527, the troy ounce became the official standard measurement for gold and silver in Britain, and the US followed suit in 1828.

Prior to the troy ounce, the tower pound and tower ounce (based on a standard scale stored at the Tower of London) were used to measure the purity of monetary pounds, ounces and pennyweights from about 800AD.

One Tower Pound = 240 Tower Pennyweights = 349.914114 grams = 5400 barleycorns or 7680 (wheat) grains

One Tower Ounce = 20 Tower Pennyweights = 29.1595095 grams = 450 barleycorns or 640 (wheat) grains

One Tower Pennyweight = 1.457975475 grams = 22.5 barleycorns or 32 (wheat) grains

But by 1527, the pennyweight no longer bore any semblance to the silver content of the English penny.

Today all coins consist of copper.

In the Troy system, 

One Troy Pound = 240 Pennyweights = 373.241722 grams = 5760 grains
One Troy Ounce = 20 Pennyweights = 31.1034768 grams = 480 grains
One Troy Pennyweight = 1.55517384 grams = 24 grains

Note, one Grain (or barleycorn) =  64.79891 Milligrams

While troy weights were used for precious metals, avoirdupois weights were used for everything else.

One Avoirdupois Pound = 16 ounces = 453.59237 grams = 7000 grains
One Avoirdupois Ounce = 28.349523125 grams = 437.5 grains

Now for Carats

When weighing diamonds, the modern carat weighs 200 milligrams.

But from about 300 AD in Rome until about 1000 AD in Constantinople (Istanbul), the solidus gold coin that was issued to soldiers (known as the soldo in Italy and the sou in France) contained about 4.536 grams of gold, equal to 24 carats, with each carat thus weighing 189 milligrams.

Later when measuring purity in gold coins, carats refer to level of fineness, with 24 carat meaning 99.9% fine.

23 carat = 95.8333% fine
22 carat = 91.6666% fine
21 carat = 87.5% fine
20 carat = 83.3333% fine etc

In England the carat was then divisible into four grains, and the grain was divisible into four quarts.
So a gold alloy of fineness 99.2 per cent purity could have been described as being 23-carat, 3-grain, 1-quart gold where 3-grain and 1-quart equalled 13/16 of 4.1666% or 3.3853625% approximately.

** End notes