| Reference | Pints (pt) | Microliters (µL) |
|---|---|---|
| A teaspoon | 0.00879877 pt | 5000 µL |
| A can of soft drink | 0.580719 pt | 330000 µL |
| A wine bottle | 1.31982 pt | 750000 µL |
| A bathtub | 263.963 pt | 150000000 µL |
| An Olympic swimming pool | 4399385 pt | 2.5 × 1012 µL |
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 microlitre is a unit of volume equal to one millionth of a litre, written µL. It is exactly one cubic millimetre. It is the working unit of the molecular biology laboratory and of clinical haematology, and almost every quantity in modern biological science that is not a mass or a concentration is a number of microlitres.
Pipetting is what defines it in practice. Adjustable micropipettes are named after their maximum volume in microlitres, so a bench holds instruments of 2, 20, 200 and 1000, and a researcher's day consists largely of moving microlitres from one tube to another. A polymerase chain reaction is typically assembled in twenty to fifty microlitres, and a whole experiment may use less liquid than a teaspoon holds.
Blood counts are reported against it. White cells number between four thousand and eleven thousand per microlitre in a healthy adult, platelets between a hundred and fifty thousand and four hundred thousand, and red cells about five million. Those reference ranges are among the most widely used numbers in medicine, and all of them are counts within a cube one millimetre on a side.
Diagnostics has driven the volumes down. A modern blood glucose meter needs about half a microlitre of blood, which is why a finger-prick sample is enough where an earlier generation of instruments needed a syringe. Point-of-care analysers, lateral flow tests and microfluidic cartridges are all designed around the smallest sample a patient can be asked to give.
Analytical chemistry uses the same scale. Injection volumes in liquid chromatography run from one to twenty microlitres, gas chromatography uses one or less, and mass spectrometry samples are prepared in tens. Because the analytical signal depends on the amount injected, the accuracy of a microlitre-scale measurement sets the accuracy of the whole result.
The identity with the cubic millimetre is a genuine convenience. Haematology once reported cell counts per cubic millimetre and now reports them per microlitre, and the numbers did not change at all, because the two units are the same. That is a rare case of a unit change in medicine that required no re-education and produced no errors.
One microlitre equals 1 cubic millimetre, 0.001 millilitres, 0.000001 litres, or about 0.0000338 US fluid ounces.