What this calculator does
Fish tanks are almost always sold and discussed in gallons in the US and in litres almost everywhere else, and the two numbers do not translate at a glance. This calculator converts a tank size between US gallons and litres in either direction, so a 10 gallon fish tank in litres, a 20 gallon fish tank in litres, or a litre figure converted back to gallons, are all the same lookup.
The conversion is a fixed, exact factor: one US gallon is 3.78541 litres, always, regardless of the tank’s shape. That is different from working out a tank’s actual capacity from its length, width and height, which needs all three dimensions and the tank’s shape, not just one number.
The formula
Multiply a US gallon figure by 3.78541 to get litres, or divide a litre figure by 3.78541 to get US gallons. Both directions use the same fixed factor, since it is an exact unit conversion rather than an estimate.
| Term | Meaning |
|---|---|
| US gallon | The standard gallon used for fish tank sizing in the US, equal to 3.78541 litres. |
| UK/Imperial gallon | A larger gallon, 4.54609 litres, used in the UK and some other countries; shown alongside for reference. |
| Litre (L) | The metric unit of volume used for fish tank sizes almost everywhere outside the US. |
The inputs explained
| Field | What to enter |
|---|---|
| Tank size | The tank size to convert. |
| Convert | Which direction to convert: US gallons to litres, or litres to US gallons. |
When to use it
Buying equipment sized in the other unit
A filter, heater or CO2 system rated in litres needs to be matched against a tank listed in gallons, or the reverse, and getting this wrong can under- or over-size the equipment for the actual water volume.
Comparing tanks from different regions
Aquarium forums and product listings mix gallons and litres depending on where they are from, so converting a specific tank size lets you compare it directly against local standard sizes.
Working out dosing and water changes
Medication, fertiliser and dechlorinator doses are usually specified per litre or per gallon, so knowing the tank’s size in the unit the dosing instructions use avoids a dosing error.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do common US fish tank sizes come to in litres?
A range of standard US tank sizes converted to litres.
| Tank size | In litres | In UK/Imperial gallons |
|---|---|---|
| 5 gal | 18.9 L | 4.16 imp gal |
| 10 gal | 37.9 L | 8.33 imp gal |
| 20 gal | 75.7 L | 16.65 imp gal |
| 29 gal | 109.8 L | 24.15 imp gal |
| 55 gal | 208.2 L | 45.80 imp gal |
| 75 gal | 283.9 L | 62.45 imp gal |
What do common metric tank sizes come to in US gallons?
A range of round litre figures converted to US gallons.
| Tank size | In US gallons | In UK/Imperial gallons |
|---|---|---|
| 20 L | 5.28 gal | 4.40 imp gal |
| 40 L | 10.57 gal | 8.80 imp gal |
| 60 L | 15.85 gal | 13.20 imp gal |
| 100 L | 26.42 gal | 22.00 imp gal |
| 200 L | 52.83 gal | 43.99 imp gal |
Questions
How many litres is a 10 gallon fish tank?
A 10 gallon fish tank, using the US gallon, is about 37.9 litres. Enter 10 with the gallons-to-litres direction selected to get the precise figure, or any other tank size.
How many litres is a 20 gallon fish tank?
A 20 gallon fish tank, using the US gallon, is about 75.7 litres. The same 3.78541 litres per gallon factor applies at every tank size.
Is a UK gallon the same as a US gallon for fish tanks?
No. A UK/Imperial gallon (4.54609 litres) is about 20% larger than a US gallon (3.78541 litres), so a tank advertised in Imperial gallons holds noticeably more than the same number of US gallons would suggest. Check which gallon a listing is using before comparing tank sizes.
Does this tell me the capacity of a tank from its length in feet?
No, not on its own. Overall tank capacity depends on the width and height as well as the length, and those vary too much between tank shapes to convert reliably from a single length measurement. This calculator converts a known gallon or litre figure; for a length-based estimate, the tank’s actual dimensions are needed.
To work out a tank’s volume in litres directly from its length, width and height, see the volume in litres calculator. For a general tank or pool capacity estimate, use the tank and pool volume calculator.