Aquarium Volume Calculator
Calculate your aquarium's volume in gallons and liters for rectangle, bowfront, cylinder, hexagon, and corner tanks. Free and instant results.
Tank Volume
Gross Volume
19.9 gal
Usable Volume (~90%)
18.0 gal
Volume in Liters
75.5 L
Usable volume accounts for substrate, decorations, and equipment displacement (~10% reduction).
Recommended Tanks for This Size
How It Works
This calculator uses standard geometric formulas to compute the water volume of your aquarium based on its shape and dimensions. Enter your tank measurements in inches or centimeters and select the shape that best matches your tank.
- Rectangle: Length x Width x Height
- Bowfront: Same as rectangle but with a 10% increase for the curved front panel
- Cylinder: Pi x (Diameter/2)^2 x Height
- Hexagon: (3 x sqrt(3) / 2) x Side^2 x Height
- Corner (quarter circle): (Length^2 / 2) x Height
The “usable volume” estimate subtracts roughly 10% from the gross volume to account for substrate, decorations, and equipment that displace water in a real setup.
Frequently Asked Questions
How do I calculate aquarium volume for a rectangular tank?
Multiply the length x width x height in inches, then multiply by 0.004329 to convert cubic inches to US gallons. For example, a 24" x 12" x 16" tank holds about 20 gallons.
What is the difference between gross and usable volume?
Gross volume is the total water capacity of the empty tank. Usable volume (about 90% of gross) accounts for space taken up by substrate, decorations, equipment, and the fact that you never fill a tank to the absolute brim.
How do I measure a bowfront aquarium?
Measure the length across the straight back panel, the width from the back to the deepest point of the bow, and the height from inside bottom to the top rim. Bowfront tanks hold roughly 10% more water than a rectangle with the same dimensions.
Why does tank shape matter for fish keeping?
Surface area affects gas exchange (oxygen in, CO2 out). A long, shallow tank provides more surface area per gallon than a tall, narrow one, which generally supports more fish. This is why a 20-gallon long tank can house more fish than a 20-gallon tall.
How do I convert gallons to liters for my aquarium?
Multiply US gallons by 3.785 to get liters. For example, a 20-gallon tank holds about 75.7 liters. Our calculator does this conversion automatically.