If you keep fish, you will meet two gallons. One is American and one is British, and they are not the same size. Care sheets, forum posts, filter ratings and tank labels use “gallons” without saying which, and the gap is 20%. A tank that is 55 gallons in one system is 25% bigger in litres than a tank that is 55 gallons in the other. If you are working out a water change, a treatment or the weight on a stand, using the wrong gallon is a straightforward error, and an easy one to make. This guide gives the two definitions, the conversion both ways, a table of common tanks, guidance on how to tell which gallon you are looking at and what to do when you cannot. The Aquarium Volume Calculator shows litres and US gallons.
The two definitions
| Unit | Litres | Kilograms of fresh water (about) | Used in |
|---|---|---|---|
| US liquid gallon | 3.785411784 | 3.785 | United States |
| UK (imperial) gallon | 4.54609 | 4.546 | United Kingdom and some other countries historically |
The ratio is 4.54609 ÷ 3.785412 = 1.2009. So a UK gallon is 20.1% bigger than a US gallon, and a US gallon is 83.3% of a UK gallon.
| Convert | Multiply by |
|---|---|
| Litres to US gallons | 0.264172 |
| Litres to UK gallons | 0.219969 |
| US gallons to litres | 3.785412 |
| UK gallons to litres | 4.54609 |
| US gallons to UK gallons | 0.832674 |
| UK gallons to US gallons | 1.200950 |
Common tanks in litres and both gallons
| Litres | US gallons | UK gallons |
|---|---|---|
| 20 | 5.3 | 4.4 |
| 40 | 10.6 | 8.8 |
| 64.8 (60 × 30 × 36 cm) | 17.1 | 14.3 |
| 100 | 26.4 | 22.0 |
| 182.3 (90 × 45 × 45 cm) | 48.1 | 40.1 |
| 300 (120 × 50 × 50 cm) | 79.3 | 66.0 |
The tank dimensions give the volume in litres, which is the same everywhere. Only the gallon count changes. See how to calculate aquarium volume for the formulas.
The same label, two volumes
A tank sold as “55 gallon” means different volumes depending on the gallon.
| Label | If US gallons | If UK gallons | Difference |
|---|---|---|---|
| 10 gallon | 37.9 L | 45.5 L | 7.6 L |
| 20 gallon | 75.7 L | 90.9 L | 15.1 L |
| 29 gallon | 109.8 L | 131.8 L | 22.0 L |
| 55 gallon | 208.2 L | 250.0 L | 41.8 L |
| 75 gallon | 283.9 L | 341.0 L | 57.1 L |
For the 55 gallon label, the gap of 41.8 litres is as large as a whole 40 litre tank. If you took a US care sheet that says “change 25% of a 55 gallon tank” and applied it to a 250 litre tank, you would think 13.75 US gallons is 52 litres, when 25% of 250 litres is 62.5 litres. The mistake is small once, and it repeats every week.
How to tell which gallon you have
- Look for litres. Many labels give both. A volume in litres is unambiguous.
- Check the country. Equipment made for the American market uses US gallons; in the UK, tanks are almost always sold in litres, with older references in imperial gallons.
- Check the dimensions. Calculate the volume from the inside measurements and compare it with the label. A “55 gallon” tank that measures 48 × 13 × 21 inches inside holds 48 × 13 × 21 ÷ 231 = 56.7 US gallons by geometry, so it is US. One that measures 120 × 50 × 50 cm holds 300 litres by geometry, which is 66 UK gallons or 79 US gallons, so the label “66 gallon” would be imperial.
- Ask the maker. A short email settles it.
- Measure. Fill the tank from a measured container.
When the tank does not match any label, trust the geometry and the water you add.
What goes wrong when you mix them
| Task | Effect of using the wrong gallon |
|---|---|
| Water change of a given percentage | The volume replaced is 20% off |
| Adding a treatment by volume | The amount is 20% off; follow the label and use the real volume |
| Choosing a filter by tank gallons | The rated turnover differs by up to 20% |
| Estimating water weight | 3.785 kg against 4.546 kg per gallon |
| Comparing prices per gallon | One seller’s gallon is bigger |
Filter ratings, such as “for tanks up to 75 gallons”, are written with a gallon in mind. When the maker is in a country that uses one gallon, assume it unless told otherwise, and check the litre figure if there is one. See aquarium filter turnover for the arithmetic.
Water weight
A tank holds about 1 kg of water per litre. In gallons:
| Tank | Water weight (to the brim) |
|---|---|
| 55 US gallons (208.2 L) | 208 kg (459 lb) |
| 55 UK gallons (250.0 L) | 250 kg (551 lb) |
(The US gallon of water is about 8.35 lb and the UK gallon about 10.02 lb.) If a tank is put on a stand rated by weight, that 92 lb difference matters. See how much a fish tank weighs and aquarium stand load.
Working in litres
The simplest way to avoid the problem is to work in litres whenever you can, and convert a care sheet’s gallons to litres once, at the start, noting which gallon you assumed.
| Care sheet says | Assume | Litres |
|---|---|---|
| “Change 20% weekly of a 30 gallon tank” (US source) | US | 113.6 L; 20% = 22.7 L |
| The same sentence (UK source) | UK | 136.4 L; 20% = 27.3 L |
Then do all later sums in litres. The Aquarium Water Change Calculator works in litres and gallons.
A worked example
Cara has bought a second-hand tank described as “60 gallon” with no dimensions. She measures the inside: 122 × 46 × 51 cm. She also finds a UK-made filter marked “for tanks up to 70 gallons”.
| Step | Working | Result |
|---|---|---|
| Volume from geometry | 122 × 46 × 51 ÷ 1,000 | 286.2 L |
| In US gallons | 286.2 ÷ 3.785 | 75.6 |
| In UK gallons | 286.2 ÷ 4.546 | 63.0 |
| Which label fits? | “60” is closest to 63.0 | Probably UK gallons, with the water level a bit below the top |
| Water at 90% fill | 286.2 × 0.9 | 257.6 L |
| Filter limit if UK gallons | 70 × 4.546 | 318 L |
| Filter limit if US gallons | 70 × 3.785 | 265 L |
Either way, the filter’s limit is above the real water volume of 258 litres. Cara decides to work in litres from now on, writes 258 L on the tank and gets on with it.
Mental shortcuts
For a quick estimate, you can divide litres by 4 for US gallons (the exact divisor is 3.785) and by 4.5 for UK gallons (4.546).
| Litres | Divide by 4 | Exact US | Divide by 4.5 | Exact UK |
|---|---|---|---|---|
| 40 | 10.0 | 10.6 | 8.9 | 8.8 |
| 100 | 25.0 | 26.4 | 22.2 | 22.0 |
| 200 | 50.0 | 52.8 | 44.4 | 44.0 |
Dividing by 4 understates US gallons by about 5%, so use it only for a first look. Dividing by 4.5 is within 1% for UK gallons. For anything that matters, such as a treatment or a water change, use the exact factor or work in litres.
Ordering equipment from abroad
A filter, heater or light sold in another country may list its tank size in the gallons of that country. If it says “up to 75 gallons” and you have a 250 litre tank, the safe reading depends on which gallon: 75 US gallons is 284 litres and 75 UK gallons is 341 litres. A 250 litre tank is within both, but a 300 litre tank is within only the UK reading. When you cannot tell, ask the seller, or choose a model that lists litres.
Common mistakes
- Assuming “gallon” means one thing worldwide.
- Applying a US care sheet to a UK-gallon label.
- Not noting which gallon you assumed.
- Mixing gallons and litres in the same sum.
- Trusting a label without checking the inside dimensions.
- Using gross volume instead of water volume.
- Forgetting the weight difference.
Quick summary
- A US gallon is 3.785 litres; a UK gallon is 4.546 litres, 20% more.
- A “55 gallon” label means 208 litres (US) or 250 litres (UK).
- A 64.8 litre tank is 17.1 US gallons or 14.3 UK gallons.
- Work out which gallon from litres on the label, the country or the geometry.
- Work in litres where possible and note your assumption.
- Use the Aquarium Volume Calculator.
Frequently asked questions
How many litres are in a US gallon?
3.785 litres, exactly 3.785411784 under the US liquid gallon definition.
How many litres are in a UK gallon?
4.546 litres, exactly 4.54609.
How do I convert US gallons to UK gallons?
Multiply by 0.8327. Ten US gallons is 8.33 UK gallons.
How do I know which gallon a label means?
Check where it was made and look for a litre figure. Where it is unclear, ask the maker or measure the tank.
Does it matter for water changes and treatments?
Yes. A 20% error in the tank size is a 20% error in the amount you add or the water you replace.
How much does a gallon of water weigh?
About 3.785 kg for a US gallon and 4.546 kg for a UK gallon.
Sources
- NIST Guide to the SI (SP 811), Appendix B.8: US liquid gallon = 3.785411784 litres (exact)
- UK Weights and Measures Act 1985 (as amended): the imperial gallon is 4.54609 litres (exact)
Related calculators
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Water Weight CalculatorWork out how much the water in a fish tank weighs so you can check your stand and floor.
- Aquarium Water Change CalculatorWork out how much water to replace for the percentage change you choose.
Keep reading
- Fish Tank Size Chart: Gallons, Litres and WeightA fish tank size chart for common US gallon sizes with typical dimensions, volume in gallons and litres and water weight, plus how to read nominal sizes.
- How to Calculate Aquarium Volume (Litres/Gallons)How to calculate aquarium volume from length, width and height in cm or inches: the formulas, US and UK gallon conversions and real water volume.
- How Much Does a Fish Tank Weigh? Full Tank WeightHow to estimate the weight of a filled fish tank: water at 1 kg per litre or 8.35 lb per US gallon, plus glass, substrate, rock and stand, with examples.
- Round vs Rectangular Tank VolumeHow to calculate round, cylinder, hexagon and rectangular aquarium volume: formulas, a cylinder table for 30 to 60 cm, a footprint comparison and real water volume.