Aquarium Volume Calculator

For a rectangular tank measured in inches, gross cubic volume is:

V = L × W × H

To convert cubic inches into US gallons, divide by 231:

Gallons = LWH/231

For metric dimensions measured in centimeters, cubic centimeters are converted into liters by dividing by 1000:

Liters = LWH / 1000

These formulas produce the gross theoretical capacity before gravel, décor, safe fill margin, and filtration hardware reduce the actual usable water volume.

Advertised aquarium size is often a gross geometric number. Actual water volume is usually smaller because tanks are not filled perfectly to the rim and because substrate, rocks, driftwood, filters, and internal equipment displace water. Practical planning should therefore distinguish between gross capacity and usable volume.

Capacity affects filtration, heater selection, medication dosing, water conditioner calculations, and stocking decisions. A tank that is mismeasured by even a modest margin can produce underpowered equipment choices or incorrect chemical dosing. This is why capacity math is central to responsible aquarium setup and maintenance. If you are interested in other liquid volume calculations like beer brewing or fermentation setups, you can also use our Homebrew ABV & Priming Sugar Calculator to plan carbonation volumes.

A rated size is a name, not a measurement. Two tanks sold as 20 gallons can have completely different footprints, and the footprint is what decides swimming room and how much surface is available for gas exchange. Measure the glass rather than trusting the label.

Dimensions are the common trade sizes; volumes and surfaces are computed from them here. Check your own tank, because manufacturers vary and glass thickness eats into the inside measurement.

The two 20-gallon rows make the point. The long has a quarter more water surface than the high for slightly less water, which is why it is the better tank for most community fish and the high is better for a tall planted scape.

Note also that the 40 breeder measures 44.9 gallons geometrically against its 40-gallon name, and the 55 measures 56.8. Nominal sizes are rounded, sometimes generously and sometimes not, which is another reason to work from the glass.

Measure the inside length, width, and water height of your tank (not the outer glass dimensions), enter them along with your unit (inches or centimeters), and set the fill percentage: how full the tank actually sits relative to the rim, typically 85–95%. The calculator returns gross volume and usable volume in both gallons and liters.

A tank measuring 36 × 18 × 16 inches is the standard 40-gallon breeder. Multiplying it out gives 10,368 cubic inches, which is 44.88 US gallons (169.9 litres), about 12% more than the name on the box. That gap is normal: the rated size is a nominal figure, and the number you should dose and stock against is the water actually in the tank, which is lower still once substrate, rocks and the gap below the rim are taken out. Allow 10–20% for those and this tank holds nearer 36–40 gallons in practice.

Check the label against the dimensions rather than the other way round. A 20-gallon long measures 30 × 12 × 12 inches and a 20-gallon high measures 24 × 12 × 16: same rated volume, different footprint, and the footprint is what determines surface area for gas exchange and swimming room.

At a practical fill level of 90% (accounting for substrate, décor, and headspace below the rim), usable water volume comes to 40.39 gallons (152.9 liters). That usable figure, not the gross or the labeled size, is what should drive heater wattage, filter turnover, and medication dosing decisions.

Once you know your usable volume, size stocking levels with the Aquarium Bioload & Fish Stocking Capacity Calculator, which uses tank volume and shape directly in its filtration and stocking math. For a planted tank, your usable volume also scales the starting bubble-rate suggestion on the Aquarium CO2 Injection Calculator.

This volume is what every ammonia dose during cycling is calculated against. The Fish Tank Cycling Time Calculator uses it to work out the dose and to estimate how long the tank will take to cycle.

For the air-filled equivalent of the same problem, the terrarium humidity and ventilation calculator works out enclosure volume, air exchange and the water a vivarium loses each day.