Since this comes as a surprise to many of my first-time British colleagues: US customary and Imperial use the same names, but are different units. The US customary volume units (cups, gallons, etc.) are on two scales: the "tablespoon scale" which is all powers-of-two, and the "teaspoon scale" which is a third of some nearby tablespoon scale.

I used to have a handy chart of the mapping of "prefix" to power-of-two, for 2^-7 to 2^7.

Also, the US foot was supposed to be exactly 30cm, but the French couldn't get their shit together, in time.

▲ BoxOfRain 7 months ago | parent [-]

Another fun fact is that UK gallons are based on the volume occupied by ten pounds of water. Combined with the fact there's 20 ounces in UK pint this means a fluid ounce of water weighs an ounce, and a pint of water weighs a pound and a quarter.

Not that you're very likely to encounter British fluid ounces any more, the smallest imperial unit of volume I generally run into is the half-pint.

	▲	thechao 7 months ago \| parent [-]
		Yeah. It'd've been neat if the US gallon was defined to be 256in^3 (with the US in being exactly 2.5cm). Then, it'd be exactly 4000 cm^3 (4L) to a US gallon, and the table of "prefixes" would be: `gallon 2^0 half-gallon 2^-1 quart 2^-2 pint 2^-3 cup 2^-4 gill 2^-5 jill 2^-6 -- invented half-gill ounce 2^-7 tbsp 2^-8 half-tbsp 2^-9 dram 2^-10` The "positive" values are harder; you'd have to steal/reappropriate the dry prefixes: `peck 2^1 half-bushel 2^2 bushel 2^3 half-barrel 2^4 barrel 2^5 hogshead 2^6 ??? 2^7 ??? 2^8`