| Centiliters (cL) | Gigaliters (GL) |
|---|---|
| 1 Centiliter | 1 × 10-11 GL |
| 2 Centiliters | 2 × 10-11 GL |
| 3 Centiliters | 3 × 10-11 GL |
| 4 Centiliters | 4 × 10-11 GL |
| 5 Centiliters | 5 × 10-11 GL |
| 10 Centiliters | 1 × 10-10 GL |
| 20 Centiliters | 2 × 10-10 GL |
| 25 Centiliters | 2.5 × 10-10 GL |
| 50 Centiliters | 5 × 10-10 GL |
| 100 Centiliters | 0.000000001 GL |
| Reference | Centiliters (cL) | Gigaliters (GL) |
|---|---|---|
| A teaspoon | 0.5 cL | 5 × 10-12 GL |
| A can of soft drink | 33 cL | 3.3 × 10-10 GL |
| A wine bottle | 75 cL | 7.5 × 10-10 GL |
| A bathtub | 15000 cL | 0.00000015 GL |
| An Olympic swimming pool | 250000000 cL | 0.0025 GL |
The centilitre is a unit of volume equal to one hundredth of a litre, or ten millilitres. Its symbol is cL, and the lower-case cl is also used. Unlike most centi-prefixed units it has a real commercial life, because it happens to be the size that suits drink labelling, and in several countries it is the form printed on almost every bottle.
Drink containers are its main employment. In France, Belgium, Italy and Spain a wine bottle is labelled 75 cL, a beer 33 or 50 cL, a soft drink 33 cL, and a spirits miniature 5 cL. The same containers carry 750 ml and 330 ml in Britain, Germany and most of the English-speaking world, which is why identical bottles appear to bear different numbers depending on where they were filled.
Bar measures follow the same split. A standard spirit serving is 4 cL in France and 2 or 4 cL in Germany and Austria, while Britain pours 25 or 35 millilitres and the United States uses fluid ounces. European cocktail recipes are written in centilitres throughout, so a classic proportion of six parts to three to one appears as 6 cL, 3 cL and 1 cL, which reads more cleanly than the millilitre equivalents.
There is no legal difference between the forms. European Union labelling rules accept millilitres, centilitres and litres, so the choice is a matter of national typographic habit rather than of regulation. Where centilitres are used, the number on a bottle stays between one and a hundred for every ordinary container size, which is exactly the range a prefix is supposed to deliver.
The unit is also easy to picture. Ten millilitres is two teaspoons, a generous mouthful of water, or the dose measure supplied with a bottle of cough syrup. A tablespoon is one and a half centilitres, an espresso is about three, and a wine glass poured to a standard measure is twelve to fifteen.
Converting between systems needs some care because a centilitre falls between the two customary small measures. It is 0.338 United States fluid ounces and 0.352 imperial fluid ounces, so neither reads as a round number, and drink volumes converted between Europe and North America almost always end up rounded rather than exact.
One centilitre equals 10 millilitres, 0.01 litres, about 0.338 US fluid ounces, or about 0.352 imperial fluid ounces.
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.