A pint’s a pound the world around.

Traditional American mnemonic

A pint of water’s a pound and a quarter.

Traditional British mnemonic

There’s something odd going on there, isn’t there? I learned that British mnemonic at primary school, and I can still vividly recall my first encounter with the American version—in a Robert Heinlein juvenile science fiction novel, Have Space Suit—Will Travel (1958). His protagonist, for reasons which need not detain us here, needs to fill a room with water. So (in that elaborate Boy-Scout Heinlein way) he measures the room using a combination of his feet, a dollar bill and a quarter coin. Armed with the number of cubic feet he has to fill, he estimates the flow rate of his water-source using an empty can:

The can looked like a pint and a “pint’s a pound the world ’round” and a cubic foot of water weighs (on Earth) a little over sixty pounds.

Up to that point I had been agog, but then the ould fella lost me—I could only conclude that he was really hopeless at arithmetic.

It turns out that a pint’s not a pound the world around at all—only in the USA, and a few other countries in South and Central America and the Caribbean. But that’s a few more than play in the “World Series” baseball championship (USA and Canada for that one), so I suppose we can’t complain.

This is a fundamental difference between the measurement standard in the United States (US customary units) and the United Kingdom (British Imperial units)—we have volume and weight units with the same names, but different magnitudes. And that’s what this post is about.

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First, pounds. Our word pound comes from Latin libra pondo, which takes a bit of explaining. The Romans used the word libra to designate a set of balance scales (as in the name of the zodiacal constellation), but also one of the standard weight used on those scales, which we now call the “Roman pound”. Pondo is the adverb from pondus, “weight”, so libra pondo means “a pound in weight”. Unfortunately, we derived the word pound from the pondo bit, but derived the abbreviation, lb., from the libra bit.

The Roman pound weighed about 0.329 kg, and variants of that became established all across the old Roman Empire. In Britain alone there were multiple versions of the pound, used in different regions and for different purposes.

One of the more important pounds in England was the Saxon pound, equivalent to about 0.35 kg. A Saxon pound of silver was enough to make 240 silver pennies. An Anglo-Norman penny was called a sterling, perhaps from an Old English word meaning “bearing a star”, because of the star stamped on some of these coins*. So 240 silver pennies were a pound of sterlings, which is the origin of the unit of British currency, the pound sterling. (And older British readers will recall that, in pre-decimal times, there were still 240 old pennies in a pound sterling.) Like the pound weight, the pound sterling took its abbreviation from Latin libra—the £ symbol is just an ornate capital L. A reference for this important coinage weight was kept in the Tower of London, which accommodated the Royal Mint for five hundred years—and so it acquired the name tower pound.

Another pound took its name from the twice-yearly fairs held in the French city of Troyes. There seems to have been a standard weight used in Troyes to measure out precious metals and stones, and the Troyes fairs were so important for international trade that the troy pound was adopted as a standard weight in Britain, too.

The troy pound is still in use today for weighing precious metals, and it’s equivalent to about 0.373 kg. It’s divided into twelve troy ounces, and our word ounce derives from Latin uncia, “a twelfth part”. So when you read on the reverse of a South African Krugerrand coin that it contains “1OZ FINE GOLD”, that’s a troy ounce.

Apothecaries adopted the troy ounce as a standard weight for measuring drugs, subdividing it into grains, scruples and drachms^†, which I might write about on another occasion.

But the pound that is most familiar today, in the USA and UK, is the avoirdupois pound, of about 0.454 kg. That word avoirdupois is a corrupted^‡ version of seventeenth-century French aver de pois, “goods of weight”. It’s divided into sixteen ounces, in a departure from the etymology of “ounce”. This was used to measure all large weights, and it’s the pound that became the standard adopted by the British Weights and Measures Act of 1824, which gave the world (or at least the British Empire) British Imperial units.

The American and British pounds are identical. Unfortunately, there are problems with larger multiples. An American hundredweight is (intuitively enough) one hundred pounds. But elsewhere in the world, a hundredweight is generally equal to 112 pounds. In both systems, it takes twenty hundredweight^§ to make a ton, which makes the American ton smaller than a British Imperial ton, too. So if we want to be precise, we need to talk about short hundredweights and short tons (907.185 kg) for the American version, and long hundredweights and long tons (1016.05 kg) for the Imperial version.

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Now, pints. After the above discussion, you’ll be wearily unsurprised to learn that Britain also once hosted a selection of different pint measures, only one of which eventually found its way into the British Imperial system.

The volume of the pint is tied to that of the gallon—eight pints to the gallon. No-one seems to be entirely sure of the origin of either word.

Between about the fifteenth and nineteenth centuries, England (and later Britain) used an ale gallon of 282 cubic inches (4.621 litres), a wine gallon of 231 cubic inches (3.785 litres), and a Winchester gallon of 268.8 cubic inches (4.405 litres). The first two are self-explanatory; the Winchester gallon was used to measure pourable dry stuff, like grain, and was originally defined as the volume occupied by eight Troy pounds of grains of wheat, taken from the middle of the ear^‖. (The Scots and Irish had their own definitions for their own gallons, but that’s probably a diversion too far, even for me.)

The Weights and Measures Act of 1824 did away with all these gallon variants, and defined an imperial gallon as being the volume occupied by ten avoirdupois pounds of water at 62°F, which turns out to be 277.42 cubic inches (4.546 litres). Which is all fine, except you’ll note that it happened some time after the United States became an independent country.

The USA decided to base the US customary liquid gallon on the English wine gallon^¶ of 231 cubic inches, which corresponds to almost exactly eight pounds of water. With eight pints to the gallon, the ten-pound imperial gallon accounts for the British mnemonic at the head of this post, and the eight-pound US gallon accounts for the American version.

And that’s why pints and pounds (not to mention hundredweight and tons) are so confusing.

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* The Anglo-Norman silver penny seems to have been considered of good quality as currency—hence the development of the adjective, sterling, to denote something of good quality—a person of “sterling character”, for instance.
^† Drachm is pronounced, and sometimes spelled, “dram”. As a small measure, it gave us the word dram for a small drink, though no-one would thank you for a literal dram of whisky, being an eighth of an ounce—equivalent to a couple of thimblefuls.
^‡ In the words of the Oxford English Dictionary: The best modern spelling is the 17th c. averdepois; in any case de ought to be restored for du, introduced by some ignorant ‘improver’ c 1640–1650.
^§ The plural of hundredweight is hundredweight.
^‖ The grains of wheat that defined the Winchester gallon are the same grains that apothecaries used as their smallest measure of weight. The measurement system of Winchester bushels, pecks, gallons and quarts was so named because the standard reference measures were kept in the city of Winchester.
^¶ The USA also adopted the Winchester bushel as a measure for dry goods—thereby adopting two standards of volume measurement that the British government very soon legislated out of existence. But I’m sure that wasn’t because of any sort of peevishness on the part of the Brits. Almost entirely sure.