Volume in litres and gallons from your tank's dimensions. Choose the shape and enter the measurements in centimetres.
Almost every next decision depends on the true water volume, not the size printed on the box. Substrate, rock and the gap below the rim commonly take 10 to 15% off. Once you have the net figure you can size a heater, work out doses, and check stocking. From here, try the heater wattage calculator, the stocking calculator and, for planning fish numbers, how many fish you can keep.
For a rectangular tank, multiply length by width by height in centimetres and divide by 1000. This calculator handles rectangle, cube, cylinder, hexagon, bowfront and corner shapes and also gives gallons.
Gross is the full internal box; net is what actually holds water after substrate, rock and the rim gap, usually about 10% less. Use the net figure for stocking, dosing and heater sizing.
One US gallon is 3.785 litres and one UK gallon is 4.546 litres. Both are shown alongside the litre figure.
Litres = the internal volume in cm³ divided by 1000. Net water allows roughly 10% for substrate, rock and the gap below the rim. Bowfront and corner figures are approximations because the curved front is not a simple shape. 1 US gallon = 3.785 L, 1 UK gallon = 4.546 L.