Aquarium Volume Calculator
Find a fish tank's volume in gallons and liters.
Results update as you type.
How it works
Knowing your aquarium's true volume is essential for stocking fish, dosing treatments, and choosing a filter and heater, and this calculator finds it from the tank's dimensions. You enter the length, width (front-to-back depth), and height in inches, and it returns the volume in gallons and liters plus the weight of the water. The calculation is the box volume in cubic inches divided by 231, the number of cubic inches in a US gallon. This gives the tank's full geometric capacity; the actual water volume is a bit less because gravel, rocks, decorations, and the gap below the rim all displace or exclude water, so many aquarists estimate real volume at roughly 85 to 90 percent of the calculated figure. Volume matters for the stocking guideline of about one inch of fish per gallon (a rough rule with many exceptions), for correctly dosing water conditioners and medications that specify amounts per gallon, and for sizing equipment. The water weight is important too: a filled tank is heavy, so the stand and floor must support it. Enter your tank's length, width, and height to find its volume and water weight.
Gallons = (length × width × height) ÷ 231
A tank's volume in cubic inches is its length times width times height. Since one US gallon equals 231 cubic inches, dividing by 231 gives gallons. Multiplying gallons by 3.785 gives liters, and by about 8.34 gives the weight of the water in pounds.
Source:Volume geometry
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter tank length, tank width (depth) and tank height.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Volume updates instantly as you type — no button to press.
Frequently asked questions
How do I calculate my aquarium's volume?
Multiply the tank's length, width, and height in inches to get cubic inches, then divide by 231 to get US gallons. A 24-by-12-by-16-inch tank holds about 20 gallons. The calculator also gives liters and water weight. Remember the real water volume is slightly less once substrate and decorations are added.
Why is my tank's actual water volume lower?
The geometric calculation gives the full box capacity, but gravel, rocks, decorations, and the inch or two of air left below the rim all reduce the water it actually holds. Most aquarists estimate real volume at about 85 to 90 percent of the calculated figure, which matters when dosing treatments precisely by volume.
How many fish can my aquarium hold?
A common starting guideline is about one inch of adult fish per gallon of water, but this is only a rough rule. Body mass, waste output, swimming space, and species compatibility matter far more. Heavily built or messy fish need much more room, so research each species' needs rather than relying solely on the inch-per-gallon estimate.
How much does a full aquarium weigh?
Water weighs about 8.34 pounds per gallon, so a 20-gallon tank holds roughly 166 pounds of water alone, plus the glass, gravel, and decorations. Larger tanks get very heavy quickly, so always place aquariums on a stand rated for the load and on a floor that can support the concentrated weight.
Does tank shape change the calculation?
This calculator is for standard rectangular tanks. For other shapes, adjust the volume formula: a cylinder uses pi times radius squared times height, and a bow-front or hexagonal tank needs its specific area times height. Once you have the volume in cubic inches, dividing by 231 always converts it to US gallons regardless of shape.