Aquarium Volume Calculator (Gallons & Liters)
Calculate tank volume in gallons and liters from dimensions. Handles rectangular, bowfront, cylinder, and hexagon shapes.
What this calculates
Fish tank stocking, heater wattage, and filter sizing all depend on accurate volume. Manufacturers' labeled gallons are often optimistic; calculating from real interior dimensions gives the true number. This calculator handles the most common tank shapes and outputs both US gallons and liters.
Formula & how it works
Rectangular: L × W × H. Cylinder: π × r² × H. Hexagon (regular): (3√3 / 2) × side² × H. Convert cubic inches to gallons (US): ÷231. To liters: ÷61.024. Subtract 5-10 % for substrate and decoration to get realistic water volume.
Worked example
Rectangular tank 36 × 18 × 18 inches. Volume = 11,664 cu in. US gallons = 11664 / 231 = 50.5 gal. Liters = 11664 / 61.024 = 191 L. Manufacturer 'standard 55-gallon' tanks actually hold ~52 gallons of water after substrate, hardware, and reduced fill line.
Frequently asked questions
Why does volume matter for stocking?
The old '1 inch of fish per gallon' rule is outdated and dangerous. Modern stocking uses behavior, oxygen needs, and waste production. But knowing real volume is the starting point for any stocking calculation.
Heater wattage?
Roughly 3-5 watts per gallon for temperate tank, 5-7 for tropical. So a 50-gallon tropical needs ~200-300 W (often split into two heaters for redundancy).
Filter turnover?
Aim for 4-6× tank volume per hour for freshwater. So 50-gallon tank needs filters rated 200-300 GPH combined. Reef tanks higher (10-20×). Filters lose flow as they clog — buy 50 % extra capacity.
How much does the substrate displace?
Sand/gravel: ~5-10 % of tank volume. Live rock in saltwater: 15-20 %. Always calculate true water volume (post-fill, post-displacement) for dosing and medication.