Aquarium Volume Calculator
Work out gallons, litres and filled weight for rectangular, cylinder, bow-front, hex and corner tanks.
Filled weight
Water change helper
Glass thickness — general guide (informational only)
These are rough industry rules of thumb based on volume and height, not an engineering calculation. Always confirm glass thickness with the manufacturer or a dedicated glass-stress calculator, especially for tall or unusually shaped tanks.
How aquarium volume is calculated
Every shape here reduces to a cross-sectional area multiplied by height, since aquarium walls are vertical. Rectangular/cube: length × width × height. Cylinder: π × (diameter/2)² × height. Half-cylinder: a cylinder sliced lengthwise, so area = π × radius² ÷ 2, using half the width as the radius. Hexagonal: a regular hexagon's area is (3√3/2) × side², times height. Corner (quarter cylinder): a quarter circle with the two straight edges as its radius, area = π × side² ÷ 4, times height.
Bow-front is approximated as a flat-sided rectangular tank plus the extra bulge, modelled as a parabolic segment: area ≈ (width × side depth) + (2/3 × width × bow depth), all multiplied by height. This is the same approximation widely used by aquarium retailers and is accurate to within a percent or two for typical bow shapes.
Worked example: a standard 55-gallon-class rectangular tank measuring 48 × 13 × 21 inches has a volume of 48 × 13 × 21 = 13,104 in³. Dividing by 231 (cubic inches per US gallon) gives 56.7 US gallons — noticeably more than the "55-gallon" label, though the true usable water volume is less once substrate, rock and the gap below the rim are accounted for. That's what the fill-to-line height field is for: enter the actual water height instead of the full tank height to get a realistic number.
Weight: water weighs about 8.34 lb (3.78 kg) per US gallon. A furnished tank — substrate, rock, equipment and the glass or acrylic itself — typically adds roughly 15% on top of the water weight, which matters for floor loading on upper stories or over long spans. Water changes are just a percentage of the current water volume: a 20% change on that 56.7-gallon tank is about 11.3 gallons (43 litres) to remove and replace.
Aquarium Volume Calculator: frequently asked questions
How do I calculate my aquarium's volume in gallons?
For a rectangular tank, multiply length × width × height in inches, then divide by 231 (cubic inches per US gallon). This calculator does that automatically for rectangular, cube, cylinder, half-cylinder, bow-front, hexagonal and corner tanks — just pick your shape and enter the dimensions.
Why is my actual water volume less than the advertised tank size?
Manufacturer "gallon" sizes are usually rounded and measured to the rim, but you never fill a tank completely — substrate, decorations, equipment and a gap below the rim all reduce the real water volume. Use the fill-to-line height field to get a more realistic number.
How much does a full aquarium weigh?
Water alone weighs about 8.34 lb per US gallon (roughly 1 kg per litre). A furnished tank with substrate, rock and the glass itself typically weighs about 15% more than the water weight alone — this calculator shows both figures so you can check floor loading.
How do I calculate a water change amount?
Multiply your tank's current water volume by the percentage you want to change. For a 55-gallon tank at 20%, that's 11 gallons to remove and replace. Use the water change helper below the volume result to convert any percentage into gallons and litres.
What glass thickness do I need for my tank?
Required thickness depends mainly on water height (which drives pressure) and tank volume. As a rough guide, tanks under about 20 gallons commonly use 3/16" glass, 20–55 gallons use 1/4", 55–125 gallons use 3/8", and larger or taller tanks need 1/2" or thicker — always confirm with the manufacturer or a glass calculator for your exact dimensions.