How it works
volume = footprint area × water height; gallons = litres ÷ 3.785411784
The calculator converts every dimension to metres, works out the water-surface footprint for the shape you picked, multiplies by the water-column height, and converts the result to litres and US gallons. A rectangular footprint is simply length × width; a bowfront adds a parabolic front segment of area two-thirds × length × bow-out depth; a cylinder is π/4 × diameter²; a regular hexagon is √3/2 × the width across the flats squared; and a corner tank is modelled as a right-triangle footprint, half × the back-wall length squared. One US gallon is 231 cubic inches, which is exactly 3.785411784 litres, so litres divided by that factor gives gallons. Filled weight uses the density of water — a US gallon weighs about 8.34 pounds — and a full, decorated tank with glass, gravel and rock runs closer to 10 pounds per gallon, which is why the stand and the floor matter. It helps to know the standard glass tank sizes: a 10-gallon "leader" is 20 × 10 × 12 inches, a 20-gallon "high" is 24 × 12 × 16 inches, a 20-gallon "long" is 30 × 12 × 12 inches, a 29-gallon is 30 × 12 × 18 inches, a 55-gallon is about 48 × 13 × 21 inches, and a 75-gallon is about 48 × 18 × 21 inches. Because nominal names round off, a "20-gallon high" actually holds just under 20 gallons brim-full and less once you leave headroom and add substrate — which is exactly why measuring beats trusting the label.
Sources
- NIST — Special Publication 811 (unit conversions) 1 US liquid gallon = 231 in³ = 3.785411784 L exactly; 1 in = 2.54 cm and 1 ft = 0.3048 m by definition.
- USGS Water Science School — Water Density A gallon of tap water at 70 °F weighs about 8.33 pounds; water density varies slightly with temperature.
FAQ
How do I calculate the volume of my fish tank?
Measure the inside length, width and height, multiply them to get the volume, then convert. For a rectangular tank in inches, length × width × height ÷ 231 gives US gallons, because one gallon is 231 cubic inches. For round, hexagonal, bowfront or corner tanks the footprint area is different, so pick the matching shape above and the calculator handles the geometry for you.
How many gallons is a 24 × 12 × 16 inch tank?
That is the classic 20-gallon "high" footprint. 24 × 12 × 16 = 4,608 cubic inches, and dividing by 231 gives 19.95 gallons — which is why it is sold as a 20. Fill it to about 95% and drop in an inch of substrate and the real water volume is closer to 18 gallons, the number that actually matters for stocking and dosing.
Why is my tank less than its labelled size?
Nominal tank names are rounded and measured brim-full to the very top edge. In practice you leave an inch or two of headroom so water does not slosh out, and substrate, rock and equipment all displace water. The net water volume the calculator reports — after fill level and substrate — is usually 10 to 20 percent below the number on the box.
How much does a full aquarium weigh?
Water alone weighs about 8.34 pounds per gallon, so a 55-gallon tank holds roughly 460 pounds of water. Add the glass, gravel, rock and equipment and a set-up tank runs closer to 10 pounds per gallon — about 550 pounds for that 55. Always confirm the stand is rated for the load and that the floor can carry it, especially upstairs.
Do I subtract substrate and rock from the volume?
For the tank size you quote, no — that is the geometric capacity. For dosing medication, fertiliser or salt, yes: use the net water volume, because substrate and large rocks displace water and a dose calculated on the full figure will be too strong. The calculator shows both, so use the net figure whenever you are adding something to the water.
How accurate is the bowfront and corner estimate?
Bowfront and corner tanks are approximations because their curves and bevels vary by manufacturer. The bowfront adds a parabolic front segment and the corner uses a right-triangle footprint, both of which land within a few percent of the true volume for typical tanks. For dosing precision on an oddly shaped tank, fill it with a measured jug once and note the real figure.
Volumes and weights are estimates from the dimensions you enter and assume flat internal faces; bowfront and corner shapes are approximated. Net water volume depends on substrate, rock and equipment. Always confirm your stand and floor can carry a filled tank. General guidance, not structural advice.