Aquarium Volume Calculator

Background.

A tank sold as a 75-gallon, the familiar 48 × 18 × 21-inch box, rarely has 75 gallons of water in it once it is running. The name describes the glass box, not the water. The walls take up some of the outside measurement, the water line sits an inch or two below the rim, and a bed of sand, a pile of rock and a canister intake all occupy space that water would otherwise fill. Set up with an ordinary substrate and some hardscape, a tank like that might hold around 70 gallons.

The difference matters most when something is dosed by the gallon. Water conditioner, fertilizer, salt mix and above all medication are measured against the volume of water they will end up in. Dose a 70-gallon system as if it held 78 and you have put in 11% more than intended, which matters for any medication with a narrow gap between an effective dose and a harmful one. Water changes have the same arithmetic: a 25% change is a quarter of the water actually in the tank, not a quarter of the label.

The calculation itself is the volume of a rectangular box. Inside length times width times water height gives cubic inches, and dividing by 231, the exact number of cubic inches in a US gallon, gives gallons; in metric mode cubic centimeters divided by 1,000 give litres. A displacement allowance then takes off a percentage for whatever is not water, and the result is reported both ways, gross and net, so you can see how much the allowance removed. The water weight comes from the net volume at 8.33 pounds per gallon or 1 kilogram per litre, the US Geological Survey's planning figures, with an option that makes seawater 2.5% heavier.

Weight is the other reason to know the number. Water alone in a 48 × 18-inch footprint loads the stand and the floor beneath it with roughly 98 pounds per square foot before the glass, gravel and rock are added. The calculator handles rectangular tanks only; bow-fronts, cylinders, hexagons, corner tanks and the sump under a reef tank need their own geometry.

What is aquarium volume calculator?

Aquarium volume here means the water a rectangular tank holds up to its operating water line. The gross volume is the full rectangle, inside length × inside width × water height, as if the tank contained nothing but water. The net volume is what remains after a displacement allowance for substrate, rock, wood, decorations and equipment, and it is the figure to dose and plan water changes from. Both are given in US gallons, defined as exactly 231 cubic inches, and in litres, with 1 US gallon = 3.785411784 litres exactly. The British imperial gallon, 4.54609 litres, is a different and larger unit. The water weight is an estimate of the water alone, not of the filled aquarium.

How to use this calculator.

1. 01Choose the unit system before typing dimensions: inches if you measure in inches, centimeters if you measure in centimeters. Every volume is reported in both gallons and litres either way, and the gross tank volume comes back in cubic inches or cubic centimeters to match.
2. 02Enter the inside length and the inside front-to-back width. If you can only measure the outside of a glass tank, subtract the glass thickness twice from each dimension; on a 48-inch tank with quarter-inch glass the inside length is 47.5 inches.
3. 03Enter the water height from the inside floor of the tank, under any substrate, up to the water line you actually run, not to the rim. Every inch of water in a 48 × 18-inch footprint is 3.74 gallons, so this is the measurement to take carefully.
4. 04Set the displacement allowance between 0 and 50%. Use 0 for a bare quarantine tank; for a planted or rock-heavy tank, estimate the volume of substrate and hardscape, or calibrate against the water you actually add during a full refill.
5. 05Leave the density on Standard for freshwater and switch to Saltwater for a marine tank. The option changes only the two weights, by 2.5%; the volumes stay the same.
6. 06Use the net gallons or litres for dosing and water changes, and the water weight for stand and floor planning, adding the tank, stand, substrate, rock and equipment separately.

The formula.

Gross volume V = L × W × H, with L, W and H the inside length, the inside front-to-back width and the water height. In inches the product is cubic inches, and gallons = V ÷ 231, because the US gallon is defined as exactly 231 cubic inches; litres = gallons × 3.785411784, the exact conversion. In centimeters the product is cubic centimeters, litres = V ÷ 1,000, since a litre is one cubic decimeter, and gallons = litres ÷ 3.785411784. The two paths agree exactly for the same tank measured in either unit, because the inch is defined as 2.54 centimeters.

Net volume = gross volume × (1 − d ÷ 100), where d is the displacement allowance in percent, so 10% keeps 0.9 of the gross figure. The displacement output is simply gross minus net. The allowance is an assumption, not a physical constant, and it is usually the least certain number in the calculation. A 2-inch gravel bed across a 48 × 18-inch floor is 1,728 cubic inches, or 7.48 gallons of bed; water fills the gaps between grains, which in sand and gravel make up very roughly a quarter to two-fifths of the bed, so the bed itself displaces about 4.5 to 5.6 gallons, 6 to 7% of the gross volume, before any rock goes in.

Weight in pounds = net gallons × 8.33 × f, and weight in kilograms = net litres × 1 × f, where f is 1 for standard water and 1.025 for saltwater. The 8.33 pounds per gallon and 1 kilogram per litre are rounded planning values, and they are not quite consistent with each other: 1 kilogram per litre works out to 8.345 pounds per gallon. The kilogram figure is therefore about 0.2% heavier than the pound figure once converted, a gap far smaller than the uncertainty in any displacement estimate.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Geological Survey (2018). "Facts About Water." USGS Water Science School.
2. [2]U.S. Geological Survey (2018). "Water Density." USGS Water Science School.
3. [3]National Institute of Standards and Technology (2026). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.
4. [4]National Institute of Standards and Technology (n.d.). "SI Units - Volume." U.S. Department of Commerce.
5. [5]U.S. Environmental Protection Agency (2026). "Density." EPA Environmental Geophysics.