Aquarium Volume Calculator
Built by Johnny Alto · free, no sign-up · updated July 26, 2026
Enter your tank's dimensions to get its volume in US gallons and liters. Your true water volume is the number that sizes a heater, a filter, and any dosing, so we also estimate the real water volume after substrate, rock, and the fill line take their share.
Full volume0US gallons
0liters
Approx. water volume0US gallons (about 90%)
Full volume is the tank filled to the brim. Real water volume runs roughly 85 to 90 percent of that once you subtract substrate, hardscape, and the gap below the rim, so size heaters and filters off the water volume, not the box.
How the aquarium volume calculation works
A rectangular tank's volume is simply length times width times height. In inches, dividing that by 231 converts cubic inches to US gallons, because one US gallon is exactly 231 cubic inches. In centimeters, length times width times height divided by 1,000 gives liters, and multiplying liters by 0.264 gives US gallons.
A cylinder tank uses pi times the radius squared times the height. The radius is half the diameter. The math is the same after that: cubic inches divided by 231 for gallons, or cubic centimeters divided by 1,000 for liters.
Why your real water volume is lower
The brim-full number overstates how much water your tank actually holds. Substrate, rocks, driftwood, and the inch or two of air you leave below the rim all displace water. A good rule of thumb is that real water volume lands around 85 to 90 percent of the full figure, which is why heater wattage and filter flow should be matched to the water volume rather than the tank's nominal size.
Recommended gear
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Frequently asked questions
How do I calculate how many gallons my aquarium is?
Multiply the tank's length by width by height in inches, then divide by 231. For example, a 24 by 12 by 16 inch tank is 4,608 cubic inches divided by 231, which is about 20 US gallons.
How do I convert my tank size to liters?
Measure in centimeters, multiply length by width by height, then divide by 1,000. To go from US gallons to liters, multiply gallons by 3.785.
Why is my real water volume less than the tank size?
Substrate, rocks, driftwood, and the gap you leave below the rim all displace water, so a filled tank usually holds about 85 to 90 percent of its brim-full volume.
What volume should I use to size a heater or filter?
Use the estimated water volume, not the nominal tank size. Sizing to the actual water your tank holds gives you the right heater wattage and filter turnover.
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This calculator gives estimates for planning only; always confirm against your own measurements and species research. Some links are to our buying guides, which contain affiliate links. As an Amazon Associate I earn from qualifying purchases.
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