Imperial gallons and litres
One imperial gallon is exactly 4.54609 litres, by UK weights and measures law. The US gallon is about 3.785 litres, so the two differ by roughly 20 per cent, and a gallons figure quoted in the UK is almost always imperial and almost never says so.
Last reviewed 14 August 2026
The short answer
Two gallons exist and only one of them is used in the UK. The difference is large enough to matter.
| Gallon | In litres | Used in |
|---|---|---|
| Imperial | 4.54609, exactly | The UK. Older water meters, and nearly every UK pool volume. |
| US liquid | about 3.785 | The United States. American product specifications and websites. |
The ratio between them is 1.2009, so treating a US figure as imperial overstates a volume by about a fifth, and treating an imperial figure as US understates it by about a sixth. On a pool that is thousands of litres.
How this works in practice
Two places a UK reader meets gallons, and they behave differently.
- An older water meter reading in gallons. The leak test works unchanged, because it measures the difference between two readings, and only the final conversion to a billed cost needs cubic metres.
- A pool volume. Almost every UK pool is described in gallons, by the installer and by the owner, and the figure is rarely labelled imperial because in the UK it does not need to be until somebody reads an American source.
| Imperial gallons | Litres | Cubic metres |
|---|---|---|
| 1,000 | 4,546 | 4.5 |
| 5,000 | 22,730 | 22.7 |
| 10,000 | 45,461 | 45.5 |
| 20,000 | 90,922 | 90.9 |
Those matter for a pool because the loss figure is derived from the volume, so an error in the units carries straight through. work out your pool surface area in square metres calculates the volume from dimensions instead, which avoids the question entirely.
What changes the answer
Three things, and the first is the only one that changes it by a lot.
- Which gallon. About a fifth, every time, in whichever direction the mistake goes.
- Whether the figure was a nominal capacity or a measured one. A pool sold as a 10,000 gallon pool holds whatever it holds, and the two can differ by more than the imperial against US gap.
- Rounding. Four and a half litres to the gallon is a useful mental figure and it is about one per cent light, which is fine for a sanity check and not for a bill.
If a conclusion changes depending on which gallon was meant, the honest answer is that the figure is not precise enough to support the conclusion, and the pool should be measured rather than looked up.
Assumptions and sources
Litres in a cubic metre
Exact by definition
SI definition. The litre is defined as one cubic decimetre, so one cubic metre is exactly 1,000 litres.
Litres in an imperial gallon
Exact by definition
UK legal definition. The imperial gallon is defined as exactly 4.54609 litres in United Kingdom weights and measures law.
The imperial gallon is defined as exactly 4.54609 litres in UK weights and measures law, so the conversion is exact rather than measured. The US liquid gallon is defined exactly too, at about 3.785 litres, and is given here only to distinguish the two.
The pool sizes in the table are round imperial figures converted exactly, not measurements of any particular pool. They are there to make the scale of the difference visible.
What to do next
Two things.
- Establish which gallon a figure is in before converting it. In the UK, assume imperial unless the source is American.
- If a pool volume matters, measure the pool rather than trusting the number it was sold as. Nominal capacities and actual volumes are not the same thing.
Common questions
- How do I convert gallons to litres?
- Multiply by 4.54609 for imperial gallons. Multiply by 3.785 for US gallons. For a rough imperial conversion in your head, four and a half litres to the gallon is within one per cent.
- How many gallons in a cubic metre?
- 219.969 imperial gallons. That is not a separate definition, it is just 1,000 litres divided by 4.54609.
- How do I tell whether a figure is imperial or US?
- Usually by where it came from. A pool volume quoted by a UK installer or in a UK pool forum is imperial. A figure from an American manufacturer or website is US. If it genuinely matters and nothing says, convert both ways and see whether the difference changes your conclusion.
- Why do older water meters read in gallons?
- Because they predate metrication of water charging. The test on this site works identically on them, because it measures a difference between two readings rather than an absolute volume, but the conversion to a billed cost needs cubic metres.