| Gigaliters (GL) | Gallons (gal) |
|---|---|
| 1 Gigaliter | 219969248.299 gal |
| 2 Gigaliters | 439938496.598 gal |
| 3 Gigaliters | 659907744.897 gal |
| 4 Gigaliters | 879876993.196 gal |
| 5 Gigaliters | 1099846241.5 gal |
| 10 Gigaliters | 2199692482.99 gal |
| 20 Gigaliters | 4399384965.98 gal |
| 25 Gigaliters | 5499231207.48 gal |
| 50 Gigaliters | 10998462415 gal |
| 100 Gigaliters | 21996924829.9 gal |
| Reference | Gigaliters (GL) | Gallons (gal) |
|---|---|---|
| A teaspoon | 5 × 10-12 GL | 0.00109985 gal |
| A can of soft drink | 3.3 × 10-10 GL | 0.0725899 gal |
| A wine bottle | 7.5 × 10-10 GL | 0.164977 gal |
| A bathtub | 0.00000015 GL | 32.9954 gal |
| An Olympic swimming pool | 0.0025 GL | 549923 gal |
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.
The gallon in the imperial system is a unit of volume equal to exactly 4.54609 litres. Its symbol is gal. It contains four quarts, eight pints and 160 imperial fluid ounces, and it is about a fifth larger than the United States gallon, a difference that causes more confusion than any other single discrepancy between the two systems.
Its original definition was elegant. The Weights and Measures Act of 1824 fixed the imperial gallon as the volume occupied by ten pounds of distilled water at sixty-two degrees Fahrenheit under a barometric pressure of thirty inches of mercury. That tied liquid measure directly to the avoirdupois pound, so a gallon of water weighed ten pounds and a pint weighed a pound and a quarter. In 1985 the definition was replaced by the exact metric figure, but the old relationship still holds to within a fraction of a per cent.
Fuel economy keeps it in daily use. British and Irish drivers describe consumption in miles per gallon even though fuel has been sold in litres since the 1990s, and the figure is an imperial one. The same car scores about twenty per cent higher in British miles per gallon than in American, purely because the gallon is larger, which is why cross-Atlantic comparisons of efficiency are meaningless unless the gallon is specified.
Legal use in Britain is now narrow but real. Draught beer and cider must be sold in pints, half pints or third pints, and milk in returnable containers may be sold in pints, which keeps the whole imperial chain alive by keeping its smallest working member alive. Several Caribbean territories still sell motor fuel by the imperial gallon.
The difference from the American gallon is worth stating plainly. One imperial gallon is 4.546 litres and one United States gallon is 3.785, so an imperial gallon is 1.2 United States gallons. Recipes, fuel figures, tank capacities and chemical dilutions all break if the wrong one is assumed, and the error is large enough to matter but small enough to look plausible.
Below it the chain runs quart, pint, gill and fluid ounce, each a quarter or a fifth of the one above rather than a decimal step. That structure was designed for dividing a container by halving it repeatedly, which is a practical merit when pouring and a serious nuisance when calculating.
One imperial gallon equals 4.54609 litres, 8 imperial pints, 160 imperial fluid ounces, or about 1.201 US gallons.