Aquarium Volume Calculator

Knowing your exact aquarium water volume is essential for accurate medication and supplement dosing, sizing filters and heaters to the correct flow rate, calculating water change amounts, and determining safe stocking levels for your fish. A 10% error in volume can mean a 10% overdose or an underpowered filter — both harmful to livestock. Whether you call it fish tank volume, tank water volume, or aquarium capacity, the number you need is the interior water-holding volume, not the exterior dimensions printed on the box.

Every aquarium volume calculation starts with the same principle: measure the interior space of your tank and convert it to gallons or liters. For a standard rectangular aquarium (also called a rectangular prism), you multiply length × width × height (all in inches) to get cubic inches, then divide by 231 to convert to US gallons. If you measure in centimeters, divide the product by 1,000 to get liters. Need to switch between units? Use the aquarium volume unit converter to convert between gallons, liters, and cubic units instantly.

Shape matters because not every tank is a simple box. Bow front aquariums add a curved section that requires a half-ellipse calculation. Cylindrical tanks use π × r² × height. Hexagonal, octagonal, and pentagonal tanks each have their own polygon area formula. See the complete list of aquarium volume formulas for every shape. Using the wrong formula can produce errors of 10–30% or more, which leads to incorrect dosing, overstocking, and equipment sizing mistakes.

Always measure from the inside of the glass, not the outside. Exterior dimensions include glass thickness, which can add ½ to 1½ inches to each dimension depending on tank size. A ¼-inch glass panel on each side of a 10-gallon tank reduces actual interior width by a full half inch, costing you roughly 0.5 gallons of real capacity.

Finally, account for fill height. Most aquarists fill their tanks 1–2 inches below the rim to prevent overflow from surface agitation. If your 20-gallon tank is filled to 14 inches instead of the full 16-inch height, your actual water volume drops to about 17.5 gallons. Our calculators let you adjust for exact fill height so your results match real-world conditions. Working with a metric setup? Use our aquarium volume in litres guide for cm-based calculations.

If your setup includes a sump or refugium below the display tank, the total volume of water in your system is higher than the display tank alone. Use the aquarium sump volume calculator to add your sump and refugium volumes to the display tank for an accurate total system volume — the number you need for dosing, salt mixing, and filtration sizing.