Conversions · Jun 17, 2026 · By Fish Tank Volume Calculator Team
Fish Tank Volume in Gallons: US vs UK Explained
US gallons and UK gallons are not the same. Here is how to convert your fish tank volume correctly between litres and both gallons.
When you read that a tank is “20 gallons”, the number alone is not enough. A gallon is not a single unit, and using the wrong one can throw off your dosing, stocking and equipment choices. This guide explains how to calculate fish tank volume in gallons and convert between US and UK measures.
US gallons vs UK gallons
The two gallons are different sizes:
- 1 US gallon equals 3.785 litres
- 1 UK (imperial) gallon equals 4.546 litres
So a tank described as 20 gallons in the United States holds about 75.7 litres, while a 20 gallon tank in the United Kingdom holds about 90.9 litres. That is a difference of roughly 15 litres for the same number.
How to calculate gallons in a fish tank
- Work out the volume in cubic inches. Multiply length, width and height with all measurements in inches.
- Divide by 231 to get US gallons, or by 277.4 to get UK gallons.
For example, a tank measuring 24 by 12 by 16 inches is 24 × 12 × 16 = 4,608 cubic inches. Divide by 231 and you get about 19.9 US gallons.
Converting from litres
If you already know the volume in litres, the conversions are quick:
- Litres to US gallons: multiply by 0.264
- Litres to UK gallons: multiply by 0.220
A 100 litre tank is therefore about 26.4 US gallons or 22.0 UK gallons.
Let the calculator do it
Our fish tank volume calculator shows US gallons and UK gallons side by side, so you never mix them up. Just choose your shape, enter the dimensions, and read both figures at once.
Why the right gallon matters
Medications, water conditioners and salt mixes are dosed per gallon or per litre. Using a US gallon dose in a tank measured in UK gallons, or the other way round, can under or over treat your water. Always check which unit a product refers to before dosing, and confirm your tank volume first.