| Reference | Pints (pt) | Cubic feet (ft³) |
|---|---|---|
| A teaspoon | 0.00879877 pt | 0.000176573 ft³ |
| A can of soft drink | 0.580719 pt | 0.0116538 ft³ |
| A wine bottle | 1.31982 pt | 0.026486 ft³ |
| A bathtub | 263.963 pt | 5.2972 ft³ |
| An Olympic swimming pool | 4399385 pt | 88286.7 ft³ |
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 foot is a unit of volume equal to 1728 cubic inches, or 28.3168 litres. Its symbol is cu ft, sometimes written ft³. It is the volume of a cube one foot on each side, and it is the unit in which the United States measures natural gas, appliance capacity and air flow, three fields with almost nothing else in common.
Natural gas is its largest use by far. American gas meters record consumption in hundreds of cubic feet, bills are issued in thousands, and national reserves are quoted in trillions. Because the energy in a given volume of gas depends on its composition, utilities also publish a heating value per cubic foot, and the bill is the product of the two.
Appliances are sized in it. Refrigerator and freezer capacity in the United States is quoted in cubic feet, so a family refrigerator is about twenty and a chest freezer perhaps seven, and the same convention covers microwave ovens, washing machines and truck cargo boxes. Shoppers compare products by a number that describes usable space rather than external dimensions, which is genuinely more useful.
Air movement uses it per unit time. Fans, extractors, air conditioners and compressors are rated in cubic feet per minute, a figure abbreviated to CFM in every American catalogue. Ventilation standards are written as required air changes, which convert to cubic feet per minute once the room volume is known, so the unit sits on both sides of the calculation.
Materials and freight follow the same scale. Concrete, gravel, mulch and topsoil are sold by the cubic foot in smaller quantities and by the cubic yard in larger ones, twenty-seven cubic feet making one cubic yard. A cubic foot of fresh water weighs 62.4 pounds and holds about 7.48 United States gallons, which are the two figures most often needed alongside it.
Its metric equivalent is not memorable, at 28.3168 litres or 0.0283 cubic metres, and neither is its relationship to the cubic inch, at 1728. Those factors are why engineering outside the United States moved to metric volumes early even where lengths stayed imperial for longer.
One cubic foot equals 1728 cubic inches, about 28.317 litres, about 7.48 US gallons, or about 0.0283 cubic metres.