| Gills (gi) | Gigaliters (GL) |
|---|---|
| 1 Gill | 1.420653125 × 10-10 GL |
| 2 Gills | 2.84130625 × 10-10 GL |
| 3 Gills | 4.261959375 × 10-10 GL |
| 4 Gills | 5.6826125 × 10-10 GL |
| 5 Gills | 7.103265625 × 10-10 GL |
| 10 Gills | 0.000000001420653125 GL |
| 20 Gills | 0.00000000284130625 GL |
| 25 Gills | 0.0000000035516328125 GL |
| 50 Gills | 0.000000007103265625 GL |
| 100 Gills | 0.00000001420653125 GL |
| Reference | Gills (gi) | Gigaliters (GL) |
|---|---|---|
| A teaspoon | 0.0351951 gi | 5 × 10-12 GL |
| A can of soft drink | 2.32288 gi | 3.3 × 10-10 GL |
| A wine bottle | 5.27926 gi | 7.5 × 10-10 GL |
| A bathtub | 1055.85 gi | 0.00000015 GL |
| An Olympic swimming pool | 17597540 gi | 0.0025 GL |
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 gigalitre is a unit of volume equal to a billion litres, or one million cubic metres. Its symbol is GL. It is also exactly one cubic hectometre, which is the same quantity written in coherent SI form. It is the unit in which reservoirs, catchments and national water plans are counted, and it is where water measurement stops describing supply and starts describing landscape.
Dam storage is its principal use. Australian reservoir levels are published in gigalitres and as a percentage of a stated capacity in the same unit, so a city's water security can be read as a single number that changes week by week. Warragamba Dam, which supplies most of Sydney, holds a little over two thousand gigalitres when full, and the whole system a few hundred more.
Policy is written in it as well. The Murray-Darling Basin Plan set out to recover water for the environment in gigalitres, and every argument about that plan has been an argument about how many gigalitres should come out of irrigation and go back into rivers. Desalination plants are rated the same way: a large plant produces something like ninety gigalitres a year, which is a meaningful fraction of a city's demand.
Australians have an informal comparison for it. The volume of Sydney Harbour is taken as roughly five hundred gigalitres, and flood volumes, storage losses and river flows are described in harbours as casually as elsewhere they are described in swimming pools. Two thousand Olympic pools make one gigalitre, which is the more portable comparison for readers elsewhere.
The unit's identity with the cubic hectometre is worth holding on to. Spanish and Latin American hydrology publishes reservoir capacity in cubic hectometres, so a Spanish figure of 500 hm³ and an Australian figure of 500 GL describe exactly the same body of water, and any comparison between the two systems needs no arithmetic at all.
Above the gigalitre, water quantities pass to the cubic kilometre, which is a thousand of them, and are used for lakes, ice sheets and global budgets rather than for anything a government can manage. Below it, the megalitre takes over for individual farms, works and pipelines.
One gigalitre equals 1,000,000,000 litres, 1,000,000 cubic metres, 1 cubic hectometre, or about 264 million US gallons.