Aquarium Tank Volume Calculator.

Multiply the interior length, width, and height in inches, then divide by 231 (the number of cubic inches in one US gallon). For metric measurements, multiply length, width, and height in centimetres and divide by 1,000 to get litres. Always measure the inside of the glass rather than the outside, because trim and glass thickness inflate external dimensions.

Printed capacities are usually rounded and based on the tank's brim-full external size. Once you subtract glass thickness, leave a few centimetres of freeboard below the rim, and account for substrate, rocks, and equipment, actual water volume typically runs 10-15% below the advertised number. A 29 gallon tank with two inches of substrate and some driftwood often holds closer to 23-24 gallons.

Always dose on net water volume, never the label. Substrate and hardscape displace water, so treating a 50 gallon tank that really holds 42 gallons means you would overdose by nearly 20%. That margin matters a great deal with copper-based medications and concentrated plant fertilisers. This calculator shows an estimated net volume alongside the gross figure.

For a cylinder, use the circular area formula: pi times the radius squared, multiplied by the height. For a bowfront, the curved pane adds roughly 20-25% to the average depth, so the calculator averages the front and back depth rather than treating it as a plain rectangle. Select the matching shape above and the correct formula is applied automatically.

A common guideline is four to six times tank turnover per hour, so a 50 gallon tank suits a filter rated around 250 gallons per hour. Planted tanks often run at the lower end to avoid disturbing CO2 distribution, while messy, heavily stocked, or marine systems benefit from the higher end. The calculator reports a 5x recommendation as a starting point.

Fresh water weighs about 8.34 pounds per US gallon, or 1 kilogram per litre. Saltwater is slightly denser at roughly 8.55 pounds per gallon. Water alone is the bulk of a filled tank's mass, which is why a 55 gallon aquarium carries roughly 459 pounds of water before you add glass, substrate, and equipment. Use the <a href="/calculators/tank-weight">tank weight calculator</a> to estimate the full loaded mass.