| Reference | Quarts (qt) | Cubic decimeters (dm³) |
|---|---|---|
| A teaspoon | 0.00439938 qt | 0.005 dm³ |
| A can of soft drink | 0.290359 qt | 0.33 dm³ |
| A wine bottle | 0.659908 qt | 0.75 dm³ |
| A bathtub | 131.982 qt | 150 dm³ |
| An Olympic swimming pool | 2199692 qt | 2500000 dm³ |
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 decimetre is a unit of volume equal to one thousandth of a cubic metre, written dm³. It is the volume of a cube ten centimetres on each side, and since 1964 it has been the exact definition of the litre. The two are the same quantity, and the choice between them is the clearest example in the whole metric system of a name being chosen for what it does to a calculation.
Chemistry prefers the cubic decimetre for a specific reason. Concentration is written in moles per cubic decimetre, and every other quantity in the same calculation is expressed in metres, kilograms and seconds or their prefixed forms. Keeping volume as a length cubed means the units in an equation cancel by inspection, without anyone having to remember that a litre is a thousandth of a cubic metre.
That is why British and Commonwealth chemistry teaching writes mol dm⁻³ where American teaching writes molar. A one molar solution and a one mole per cubic decimetre solution are identical, but only the second form makes the dimensions visible. Students trained on it can check an answer by looking at the units, which is the most reliable error-catching habit in quantitative chemistry.
Gas volumes follow the same convention. One mole of an ideal gas occupies 22.4 cubic decimetres at zero degrees Celsius and one atmosphere, and about 24.0 cubic decimetres at room temperature, so a reaction stoichiometry converts to a measurable volume in one step. These are among the most quoted numbers in school chemistry, and they are almost always written in dm³ rather than in litres.
Elsewhere the litre wins easily. Nobody buys fuel, milk or paint in cubic decimetres, and no drink label carries the symbol, because the litre name is shorter, older and understood by everyone. The cubic decimetre survives where a formula rather than a customer is reading the number.
The relationship to its neighbours is worth keeping straight because of the cubing. A cubic decimetre holds a thousand cubic centimetres, not a hundred, and a thousand cubic decimetres make a cubic metre. Each step in length is a factor of ten and each step in volume a factor of a thousand, which is the source of most errors made with this unit.
One cubic decimetre equals 1 litre, 1000 cubic centimetres, 0.001 cubic metres, or about 61.02 cubic inches.