Aquarium Volume Calculator

Start with the space inside the glass

Measure the tank’s internal dimensions. An outside measurement includes the glass and overstates the space available for water. For a rectangular tank, multiply length, width and height in centimetres, then divide by 1,000 to get litres:

Volume (litres) = length (cm) × width (cm) × height (cm) ÷ 1,000

A 60 × 30 × 30 cm tank holds 54 L when filled to its internal height, before anything is placed inside. NIST’s volume guide explains the litre as a cubic decimetre; a 10 cm cube contains 1,000 cubic centimetres.

For a cube, use the same side length three times. A cylinder uses its internal diameter and height: the base area is π times the radius squared, with radius equal to half the diameter. For a regular hexagon, the base is six equilateral triangles, giving an area of 3 × √3 × side² ÷ 2. The hexagon control needs that side measurement, not the distance across opposite corners. Multiply either base area by height to obtain the volume.

Two common tank sizes, worked through

A 30 cm cube holds 27 L before displacement. Its volume is 30 × 30 × 30 = 27,000 cm³. Dividing by 1,000 gives 27 L, approximately 7.133 US gallons or 5.939 Imperial gallons. Select the 30 cm cube example to load these dimensions.

A 5 × 2 × 2 ft aquarium holds approximately 566.337 L. The three measurements give 20 ft³. One foot is 30.48 cm, so one cubic foot contains 30.48³ / 1,000 = 28.316846592 L. Multiplying by 20 gives 566.33693184 L, or approximately 149.61 US gallons. These are internal dimensions: a tank advertised as five feet long can have less internal capacity.

“2 ft × 1 ft” is missing a dimension. If those are length and width, a height of 1 ft gives about 56.634 L, whereas a height of 1.5 ft gives about 84.951 L. A “5 ft fish tank” is likewise insufficient to determine capacity without width and height.

Full capacity and the water actually in the tank

Leave the calculator in Simple mode for the geometric capacity of a full, empty tank. In Advanced mode, enter the water-surface height as a percentage of the internal tank height. All supported shapes have vertical sides, so the same percentage applies to their volume.

Suppose the 60 × 30 × 30 cm tank has a water surface 27 cm above the bottom. Its fill height is 27 / 30 × 100 = 90%, leaving 54 × 0.9 = 48.6 L of space below that surface. If submerged solids occupy 5 L, the estimated water volume is 48.6 − 5 = 43.6 L.

Enter displacement in litres. Count the volume of submerged solid material that excludes water; a bag’s bulk gravel volume also includes gaps where water can sit. The calculator cannot infer displacement from substrate depth or a photograph. If you do not know it, use zero and treat the result as volume before solids. Displacement cannot exceed the filled tank volume; tiny numerical rounding differences at equality are treated as zero water.

The diagram shades the entered fill height. It does not lower that surface when you enter displacement: the water volume changes because solids occupy some of the space beneath it. Sumps, external filters and plumbing are outside these tank measurements.

Choose the right gallon result

US liquid gallons and Imperial gallons differ. This calculator uses 3.785411784 L per US gallon and 4.54609 L per Imperial gallon. The US factor follows from 231 cubic inches per gallon and exactly 2.54 cm per inch. NIST’s conversion tables document the length and gallon conversions.

Both gallon results describe the estimated water volume. The separate full-capacity result remains in litres, making it possible to compare an empty tank with the adjusted water estimate. Values are shown to at most three decimal places; very small nonzero values use scientific notation instead of appearing as zero. Extra digits in a result do not improve the accuracy of your measurements.

Tank volume is only one part of choosing fish or building an aquarium. This tool does not determine a safe stocking count, glass thickness, support loading or suitability for a particular species. It also does not approximate bow-front tanks, fish bowls or irregular shapes.