| Reference | Gills (gi) | Fluid ounces (fl oz) |
|---|---|---|
| A teaspoon | 0.0351951 gi | 0.175975 fl oz |
| A can of soft drink | 2.32288 gi | 11.6144 fl oz |
| A wine bottle | 5.27926 gi | 26.3963 fl oz |
| A bathtub | 1055.85 gi | 5279.26 fl oz |
| An Olympic swimming pool | 17597540 gi | 87987699 fl oz |
The gill is a unit of volume equal to a quarter of an imperial pint, or exactly 142.0653125 millilitres. Its symbol is gi. It is five imperial fluid ounces, and it sits between the fluid ounce and the pint in the old British liquid chain. Its name is pronounced with a soft g, so that it sounds like the name Jill, which surprises most readers who meet it only in print.
For most of the twentieth century it governed the British pub. Spirits were served in fractions of a gill, and the fraction varied by region: a sixth of a gill in England and Wales, a fifth in Scotland, and a quarter in Northern Ireland. A traveller ordering the same drink in three cities therefore received three different quantities, all legally correct and all described by the same words.
That system ended in 1995, when the United Kingdom replaced the gill fractions with metric measures of 25 or 35 millilitres, each establishment choosing one and displaying which. The change was less disruptive than it looked, because a fifth of a gill is 28.4 millilitres and a quarter is 35.5, so the new measures sat close to the old ones and the drinks barely changed size.
Cookery kept it longer than the law did. British and Irish recipes written before the 1970s call for a gill of cream, stock or milk, and the quantity is a quarter pint, which is 142 millilitres. Anyone working from an inherited recipe book meets the word regularly, and the conversion is one of the few worth writing on the flyleaf.
The American gill is a different quantity again. It is a quarter of a United States liquid pint, therefore four United States fluid ounces or 118.3 millilitres, about a sixth smaller than the imperial gill. It appears in old American recipes and in the occasional bar manual, and the same caution applies as with the pint: the name says nothing about which system is meant.
Today the gill survives in dialect, in historical writing and in the specifications of glassware sold for period use. It is one of the clearest cases of a unit outliving its own function, kept alive by cookbooks and by the memory of how a drink used to be poured.
One imperial gill equals 142.065 millilitres, 5 imperial fluid ounces, 0.25 imperial pints, or about 1.201 US gills.
The fluid ounce in the imperial system is a unit of volume equal to exactly 28.4130625 millilitres. Its symbol is fl oz. Twenty of them make an imperial pint and 160 an imperial gallon. It is the smallest unit of the imperial liquid chain in ordinary use, and its name records exactly what it was built to do.
The name is literal. The Weights and Measures Act of 1824 defined the fluid ounce as the volume occupied by one avoirdupois ounce of water, so a fluid ounce of water weighs an ounce. The modern definition is metric and the agreement is no longer exact, but it is close: a fluid ounce of water weighs 28.4 grams against the ounce's 28.35, a difference of two parts in a thousand.
The United States fluid ounce is a different quantity, and the relationship between the two is the most confusing pair of units in either system. A United States fluid ounce is 29.5735 millilitres, so it is four per cent larger than the imperial one. Yet a United States pint contains only sixteen of them where an imperial pint contains twenty, so the American pint ends up seventeen per cent smaller. The smaller unit belongs to the larger system and the larger unit to the smaller.
That reversal has practical consequences. A recipe calling for eight fluid ounces of milk means 237 millilitres in America and 227 in Britain, which is tolerable, but one calling for a pint means 473 or 568, which is not. Cosmetics, medicines and drinks all carry both figures on internationally sold packaging for exactly this reason.
British labelling now leads with millilitres. Toiletries, spirits and soft drinks are specified in metric units, with fluid ounces printed alongside for markets that expect them, and pharmacy long ago abandoned the fluid ounce and the fluid dram in favour of millilitres. What survives is culinary and conversational rather than legal.
Where an old British recipe uses it, the conversion is straightforward as long as the system is known: multiply by 28.4 for millilitres, and remember that the pint above it holds twenty and not sixteen. Getting the pint right matters more than getting the ounce right, because the error compounds with quantity.
One imperial fluid ounce equals 28.4130625 millilitres, 1/20 of an imperial pint, or about 0.961 US fluid ounces.