Nominal aquarium size and actual water are different
An aquarium advertised by size may not contain that volume of water during normal operation. Glass thickness reduces its internal dimensions, the waterline sits below the rim, and submerged objects displace water. This calculator starts from the measured interior of a rectangular tank and accounts for the latter two reductions explicitly. It estimates the water in the display tank at the operating level. Connected sumps, external filter chambers and hoses are excluded unless you calculate their water separately and add it.
Find the filled height before subtracting decor
Water volume = inside length × inside width × (inside height − headspace) − submerged displacementMetric dimensions use centimetres, with 1,000 cubic centimetres in one litre. Imperial dimensions use inches, with 231 cubic inches in one US gallon. Displacement is entered directly in litres or US gallons, rather than as the exterior dimensions of decorations. The tool reports brimful capacity, waterline capacity before subtraction and the final estimated water volume so you can see each stage. Headspace must be smaller than the internal height, and displacement cannot exceed the waterline capacity. There is no waste or purchasing margin in this stored-water measurement.
A rectangular display tank example
A tank measuring 100 by 40 by 50 centimetres internally has a brimful capacity of 200 litres. Leaving 5 centimetres unfilled creates a 45 centimetre water depth, which holds 100 × 40 × 45 ÷ 1,000 = 180 litres before objects are added. Substrate and decoration displacing 10 litres leave an estimated 170 litres of operating water. At the approximate freshwater density of 1 kilogram per litre, that water alone weighs 170 kilograms. The glass, stand, substrate, equipment and ornaments add further mass that does not appear in the water-only result.
Measure displacement and use volume carefully
Loose substrate contains gaps that fill with water, so its external pile volume is not the same as the volume of solids it displaces. A separate displacement measurement or a supplier estimate can be more useful than subtracting the entire substrate envelope. Large rocks and sealed ornaments behave differently from porous pieces. For an established aquarium, a measured refill volume can help check the geometric estimate. This page does not recommend stocking levels or treatment doses, and tank dimensions alone cannot determine either. For any product whose quantity depends on water volume, follow its instructions and verify the actual system volume. The mass estimate is approximate freshwater mass and does not certify a floor, cabinet or tank support. Unusual shapes and bowed fronts require a different geometry from the rectangular model used here.
Frequently asked questions
Should I use the dimensions on the packaging?
Use inside measurements for the rectangular water space. Outside measurements include glass and may overstate the capacity.
Does substrate displace its whole envelope?
Usually not, because water occupies spaces between particles. Enter the displaced solid volume or a measured estimate rather than subtracting all visible substrate space.
Is an aquarium gallon US or imperial?
This calculator reports US gallons. An imperial gallon is about 4.546 litres and must be converted separately.
Does water mass include my stand and glass?
No. It includes only the approximate water mass at 1 kilogram per litre. Add all other components when assessing the total supported load.
Can this calculate fish capacity or dosage?
No. It estimates geometric operating volume only. Stocking and product use depend on additional information and their specific instructions.
Last reviewed: 2026-10-03 · How we calculate