A Pint is a Pound, the World Around
The Romans had a standard measure called a libra (“scale”, abbreviated “lb”), which historians agree was somewhere in the vicinity of 329 grams (11.6 oz). (I cannot find any clear consensus on why this particular weight was significant.) Objects in ancient Rome could be measured according to their libra pondo — “weight in libras”
Because Romans loved duodecimals for measurement, the main unit of measurement for small objects was one-twelfth of a libra — the “uncia”, which was about 27 modern grams.
Over time, the Roman system evolved into numerous other standards, depending on where and when it was standardized, and for what purpose. The basis for most of the systems remained the ounce, which was usually within about 10% of the old Roman uncia. The pound was sometimes 12 ounces (following the Roman system), sometimes 16 ounces (following powers of 2), or sometimes 15 ounces (because why the hell not).
Alongside the ounce, the “grain” — the average weight of a grain of barley — has been a standard unit of measurement since antiquity. As noted on Wikipedia: “Since the weights of these seeds are highly variable, especially … as a function of moisture,” the weight of a grain was more a matter of convention than anything else.
In the middle ages, wool merchants in England began to use the wool pound — part of the very logical system where 16 ounces made a pound, 14 pounds made a stone, and 26 stones made a woolsack. The wool pound became a key component of the “avoir du pois” (“goods of weight”) system standardized during the Plantagenet dynasty in England.
To reconcile ounces and grains, the wool ounce was defined as 437 grains, making the wool pound nominally equal to 6992 grains. This was close enough to 7000 that Queen Elizabeth I officially defined the wool pound up to 7000 grains in 1588. This round number became the most common pound in measurement, making the avoirdupois ounce fixed at 437.5 grains, or 28.3 modern grams, where it remains today.
In contrast, for precious metals, the “Troy ounce” was used. The Troy ounce was 480 grains, or 31.1 grams. There was also a unit called the “Tower pound” — often used in English coinage — which was divided into 12 Tower ounces, each of which was 20 pennyweights, each pennyweight being 32 grains. (Obviously.) The “Tower pound” was therefore 7680 grains. Now, one might logically think that that is significantly more than the avoirdupois pound of 7000 grains. But no, the English Tower system used the “wheat grain” or “Tower grain”, which was conveniently defined as 45/64 of a barley grain. So, the Tower pound of 7680 Tower grains was actually only 5400 grains (about 350 g), with each Tower ounce being 450 grains (29.2 grams).
Meanwhile, many other trade goods followed the system of the German Hanseatic League, which defined a pound as 7200 grains (466.6 grams), each made of 16 of the same 450-grain ounces as the Tower system. In England, this was called the “London pound.”
While dry goods were sold by wool pounds, liquids (okay, wine) were sold by London pounds, the standard measure being the “wine gallon” — 8 pounds of wine, or roughly 3.73 kilograms.
When Queen Anne standardized this, the wine gallon was defined as “any round vessel, commonly called a cylinder, having an even bottom, and being seven inches diameter throughout and six inches deep.” This had the effect of defining the wine gallon as 230.9 cubic inches (3784 milliliters). Accordingly, the “wine ounce” — 1/128 of a wine gallon — became 29.6 modern milliliters, where it remains to this day.
So, if you’re like me and hate the fact that a (fluid) ounce of water weighs 1.04 (avoirdupois) ounces, the reason is because in late medieval England, wool merchants and wine merchants used two different “pounds.”
Now that we’ve discussed the pound, it is logical to move onto the pint, since, as we all know, a pint is a pound, the world around. To talk about pints, we have to talk about gallons, because a pint is just one-eighth of a gallon.
We’ve already talked about the wine gallon. But while winemakers used the 231 cubic-inch wine gallon, breweries used the 282 cubic-inch ale gallon, which would be about 4.62 liters. (I have not been able to determine the origin of this, though I have seen theories based on the fact that 282 cubic inches of beer with a specific gravity of 1.01 would weigh almost exactly 10 London pounds.)
Meanwhile, the bushel was a standard agricultural unit, and it was divided into 8 dry gallons (or “corn gallons”), each of which was the volume taken up by 8 pounds of wheat. (Which pound, you ask? Good question: the Tower pound.) When Henry VII standardized this in the “Winchester system” in the late 15th century, a bushel was defined as a cylinder 18.5 inches wide and 8 inches high, with a volume of 2150 cubic inches, or 35.24 modern liters. A corn gallon, therefore, was 4.405 Liters.
So, let’s recap where we are at the time the American colonies decide they’re an independent country:
- A pint of ale would be sold in a container with a volume of 578 mL
- A pint of blueberries would be sold in a container with a volume of 551 mL
- A pint of wine would be sold in a container with a volume of 473 mL
Then, in 1824, the British had to go mess it all up again.
In the Weights and Measures Act of 1824, the UK semi-decimalized their volume system defined the imperial gallon as 10 avoirdupois pounds of water. This measure — 4.54 liters — was conveniently fairly close to the 4.62 liter ale gallon, meaning the official size of a pint of beer only dropped from 578 mL to 568 mL.
To leave the fluid ounce relatively unchanged however, the pint was defined as 20 ounces, meaning that an imperial fluid ounce is actually slightly smaller than a US fluid ounce. So, your pint of beer in England will be “20 fluid ounces” of beer, but if you measure with a US measuring cup you’ll only end up with 19.2 fluid ounces.