About this tool
The Aquarium Volume Calculator is a free, in-browser tool that works out how much water a fish tank holds — the number every heater, dose, stocking and filter decision depends on. Pick a shape preset (rectangle/cube, cylinder, half-cylinder corner, hexagon, pentagon corner, or bow-front), enter the dimensions in centimeters or inches, and it returns the volume in liters, US gallons and imperial gallons at once. It also reports a realistic fill volume after subtracting a displacement allowance (default 10%) for substrate, rock and decor. Everything runs locally, so nothing you enter is uploaded.
The method computes the internal cross-section area, multiplies by height, then converts using 1 L = 1000 cm³, 1 US gal = 3.78541 L and 1 imperial gal = 4.54609 L. Rectangle/cube area = L × W; cylinder = π(D/2)²; half-cylinder = ½·π(D/2)²; regular hexagon by across-flats width w = (√3/2)·w²; pentagon corner (two equal sides a at 90°) = ½·a²; bow-front = L × W + ⅔·L·b, where b is the bulge depth beyond the flat front. The actual fill volume is gross × (1 − displacement%).
As a worked example, a 60 × 30 × 36 cm rectangular tank has a cross-section of 1,800 cm², a gross volume of 64,800 cm³ = 64.8 L, which is about 17.1 US gallons or 14.3 imperial gallons; at a 10% displacement allowance the real water content is about 58.3 L. Getting this figure right matters because an over- or under-stated volume throws off heater sizing, medication dosing and stocking limits.