| Reference | Quarts (qt) | Cubic inches (in³) |
|---|---|---|
| A teaspoon | 0.00439938 qt | 0.305119 in³ |
| A can of soft drink | 0.290359 qt | 20.1378 in³ |
| A wine bottle | 0.659908 qt | 45.7678 in³ |
| A bathtub | 131.982 qt | 9153.56 in³ |
| An Olympic swimming pool | 2199692 qt | 152559360 in³ |
The quart in the imperial system is a unit of volume equal to exactly 1136.5225 millilitres. Its symbol is qt. It is two imperial pints and a quarter of an imperial gallon, and its name says so: quart comes from the Latin quartus, a fourth. It sits between the pint and the gallon and, in Britain, it has lost to both.
Its decline is the clearest case of a unit being squeezed out from both sides. The pint survived because it is the legal measure for draught beer and because a person can hold one; the gallon survived in fuel economy figures. The quart is neither drunk nor driven, so nothing kept it in daily speech, and British shops now describe the same quantity as a litre or as two pints.
In the United States the same name is thriving. A United States liquid quart is 946 millilitres, and milk, cream, stock, ice cream, paint and motor oil are all sold in that size. An engine oil change is quoted in quarts, five being typical for a car, and the quart carton is one of the most recognisable objects in an American kitchen.
That American survival owes something to the litre. A United States quart is only five per cent smaller than a litre, so shelf sizes barely changed when Canadian and international products moved to metric, and a shopper comparing a quart with a litre is comparing almost the same thing. The imperial quart is thirteen per cent larger than a litre, which made the same substitution less comfortable in Britain.
The dry measure adds a further complication in the United States. A dry quart is 1101 millilitres against the liquid quart's 946, and it belongs to a separate chain used for berries, produce and grain. Nothing on a label distinguishes them, so the same word describes two different quantities depending on whether the contents pour or not.
For conversion, the imperial quart is 1.1365 litres and the United States liquid quart is 0.9464. The rule of thumb that a quart is roughly a litre is safe in America and about a seventh out in Britain, which is exactly the kind of margin that turns a recipe or a chemical dilution wrong without looking obviously wrong.
One imperial quart equals 1136.5225 millilitres, 2 imperial pints, 0.25 imperial gallons, or about 1.201 US liquid quarts.
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.