Fish Tank Volume Calculator
Measure a rectangular aquarium inside length, width, and how deep you fill the water. Use the controls below for inches or centimeters — US gallons use length × width × height ÷ 231 when dimensions are in inches, and liters follow 1 US gallon = 3.785 liters. For example, 20 × 10 × 12 in is about 10.4 US gallons.
Rectangular tanks only. Measure inside length, width, and water height. Numbers stay in your browser.
US gallons
Liters
US gallons use length × width × height ÷ 231 when dimensions are in inches. Liters follow 1 US gallon = 3.785 liters.
Enter all three dimensions to see volume. Leave a field blank or zero and no result is shown.
Handy gear sized to your gallon count
- Aquarium gravel or substrate for the depth you plan on the bottom (opens in new tab)
- Aquarium heater rated for the gallon volume you calculated (opens in new tab)
- Hang-on or internal filter rated for the same gallon size (opens in new tab)
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What this covers
This tool is for standard rectangular glass or acrylic tanks with straight sides. Bow-front curves, corner pentagons, and round bowls need a different formula, so they are not included here. Salt mix, salinity, and water-change math are separate jobs — use the gallon result here when you shop for gravel depth, heater wattage, and filter flow rated to your tank size.