Fish tank calculators
Free aquarium calculators for tank volume, water changes, dosing, heaters, stocking and more. Gallons or liters, no sign up, nothing to install.
Tank setup
Water care
Why bother with the math?
Most dosing mistakes come from a wrong tank volume. A "20 gallon" tank often holds closer to 17 once gravel, rocks and the gap at the top are counted. Start with the volume calculator, then use that number everywhere else.
Common questions
- How do I calculate how many gallons my fish tank holds?
- Measure the inside length, width and water depth in inches, multiply them together, and divide by 231 to get US gallons. The tank volume calculator does this for rectangle, cylinder and hexagon tanks.
- How many liters is a 20 gallon tank?
- About 75.7 liters. Multiply US gallons by 3.785 to convert any tank size to liters.
- Are these calculators free?
- Yes. Every FishMath calculator is free, runs in your browser, and never asks you to sign up.