| Reference | Quarts (qt) | Liters (L) |
|---|---|---|
| A teaspoon | 0.00439938 qt | 0.005 L |
| A can of soft drink | 0.290359 qt | 0.33 L |
| A wine bottle | 0.659908 qt | 0.75 L |
| A bathtub | 131.982 qt | 150 L |
| An Olympic swimming pool | 2199692 qt | 2500000 L |
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 litre is a unit of volume equal to one thousandth of a cubic metre, or exactly one cubic decimetre. Its symbol is L, and the lower-case l is also permitted. It is not part of the International System of Units, but it is accepted for use alongside it, and it is almost certainly the most widely used volume unit in the world.
It was created by the French metric law of 1795 as the volume of a cube one tenth of a metre on a side. In 1901 it was redefined as the volume occupied by one kilogram of pure water at its densest, which sounds equivalent but is not: the water definition made a litre 1.000028 cubic decimetres, and for sixty-three years precise volumetric work had to state which litre was meant. The 1964 General Conference on Weights and Measures restored the cubic decimetre and the discrepancy vanished.
The symbol has its own small history. The lower-case l is easily mistaken for the digit one in many typefaces, a genuine hazard on a medicine label or a laboratory record, so in 1979 the capital L was approved as an alternative. Both are correct, and usage splits by country rather than by discipline, with North America favouring L and much of Europe still writing l.
Its everyday reach is enormous. Fuel, milk, paint, soft drinks and engine displacement are all measured in litres, fuel consumption in most of the world is stated as litres per hundred kilometres, and cooking outside the United States works in litres and millilitres. Medicine uses it for blood volume, roughly five litres in an adult, and for lung capacity, around six.
One relationship makes it exceptionally useful. A litre of water has a mass very close to one kilogram, because the kilogram was originally defined that way, so volume and mass can be swapped in the head for most practical purposes. Rainfall follows the same logic: one millimetre of rain on one square metre is exactly one litre of water.
Against imperial and United States measures it converts untidily, which is why drink sizes differ so visibly between markets. One litre is 1.76 imperial pints but 2.11 United States liquid pints, and 0.22 imperial gallons but 0.264 United States gallons, so a gallon means two noticeably different quantities depending on where the label was printed.
One litre equals 1000 millilitres, 0.001 cubic metres, about 0.264 US gallons, or about 0.22 imperial gallons.