Aquarium Volume Calculator

How it works

Rectangular: volume = L × W × H · Cylinder: volume = π × (diameter ÷ 2)² × H · gallons = cubic inches ÷ 231 · litres = cubic inches × 0.0163871 · water weight ≈ gallons × 8.345 lb

Tank dimensions are measured on the outside, but the number that matters for stocking, heaters and dosing is the actual water volume — always less than the nominal size once you subtract substrate, rocks, décor and the gap below the rim. Multiply length by width by height (or use π r² h for a cylinder) to get cubic inches, divide by 231 for US gallons, and apply a fill factor of around 90% for a realistic water figure. The weight matters too: water alone is about 8.3 lb per gallon, so a modest 40-gallon tank puts over 300 lb on the stand before you add glass, gravel and rock.

Worked example

A 24×12×16 in rectangular tank is 4,608 cubic inches. Divided by 231 that is 19.9 US gallons (about 75.5 L) full. At a 90% fill it holds roughly 18.0 gallons of actual water, which weighs about 150 lb on its own.

Frequently asked questions