| Reference | Pints (pt) | Cubic inches (in³) |
|---|---|---|
| A teaspoon | 0.00879877 pt | 0.305119 in³ |
| A can of soft drink | 0.580719 pt | 20.1378 in³ |
| A wine bottle | 1.31982 pt | 45.7678 in³ |
| A bathtub | 263.963 pt | 9153.56 in³ |
| An Olympic swimming pool | 4399385 pt | 152559360 in³ |
The pint in the imperial system is a unit of volume equal to exactly 568.26125 millilitres. Its symbol is pt. It is twenty imperial fluid ounces, an eighth of an imperial gallon, and half an imperial quart. It is the only imperial unit that most people in Britain and Ireland still use every week, and it survives because it is the legal measure for draught beer.
That legal status is precise. Draught beer and cider in the United Kingdom may be sold only in third-pints, half-pints and multiples of half a pint, and the glass itself must carry a verification mark certifying that it holds the stated measure. Publicans can be prosecuted for short measure, so the pint is one of the few units in daily life whose accuracy is actively policed.
Milk keeps the second foothold. Milk sold in returnable glass bottles may still be measured in pints, which is why doorstep deliveries are ordered by the pint while supermarket cartons carry litres. That split is a useful illustration of how metrication proceeded in Britain: it changed the shops and left the traditions alone.
The American pint is a different quantity and the difference is large. A United States liquid pint is 473 millilitres against the imperial 568, so a British pint is about a fifth bigger, and a traveller who orders a pint in New York receives noticeably less than one in Dublin. The American mnemonic that a pint's a pound the world around works only for the American pint; a British pint of water weighs a pound and a quarter.
That weight relationship was once the definition. The imperial system was built so that a gallon of water weighed ten pounds, which makes a pint weigh exactly a pound and a quarter, and the older British rhyme says so directly. Since 1985 the pint has been defined from the metric gallon instead, but the relationship still holds closely enough to be a useful check.
Below the pint the imperial chain runs to the gill at a quarter of a pint and the fluid ounce at a twentieth. Above it come the quart and the gallon. None of these steps is decimal, which is the reason the pint is now the only survivor: it is the one member of the chain that people handle directly, and the rest of the arithmetic has quietly moved to litres.
One imperial pint equals 568.26125 millilitres, 20 imperial fluid ounces, 0.125 imperial gallons, or about 1.201 US liquid pints.
The cubic inch is a unit of volume equal to 16.387064 cubic centimetres exactly. Its symbol is cu in, sometimes written in³. It is the volume of a cube one inch on each side, and 1728 of them make a cubic foot. Its most consequential role is legal rather than descriptive: the United States gallon is defined as exactly 231 cubic inches.
That definition is the reason American liquid measure is the shape it is. The figure of 231 comes from the English wine gallon standardised under Queen Anne in 1707, which the United States kept when Britain replaced its own gallons with a single imperial gallon in 1824. Every American fluid ounce, pint and quart descends from that one number, and it is why the two gallons differ by a fifth.
Engines made the unit famous. American car engines were quoted in cubic inches until the 1980s, and the numbers became names in their own right: a 350 is 5.7 litres, a 426 is 7.0, and a 302 is 4.9. Motorcycles still carry both figures, and the conversion is worth knowing, since 61 cubic inches make a litre.
Machinery uses it for swept volume. Pump and compressor displacement per revolution, hydraulic cylinder volume and metering pump strokes are all specified in cubic inches in American engineering, because the bores and strokes that produce them are already in inches and squaring or cubing an inch keeps the calculation in one system.
Freight pricing uses it in an unexpected way. American parcel carriers calculate a dimensional weight by dividing the package volume in cubic inches by a fixed divisor, commonly 139, to obtain a chargeable weight in pounds. A light bulky parcel is therefore priced by its cubic inches rather than by what it weighs, which changes how goods are packed.
The conversion to metric is exact but unhelpful. Because the inch is defined as exactly 2.54 centimetres, a cubic inch is exactly 16.387064 cubic centimetres, and no rounding is involved. That precision does not make the number easier to use, and it is the main reason engineering drawings outside the United States moved to millimetres.
One cubic inch equals 16.387 cubic centimetres, about 0.0164 litres, about 0.554 US fluid ounces, or 1/1728 of a cubic foot.