Aquarium Volume Calculator
Use our free aquarium volume calculator to convert your tank’s dimensions into an accurate water volume in gallons or liters. Also known as a fish tank volume calculator, aquarium capacity calculator or gallon calculator, it handles rectangular, cube, cylinder, half-cylinder, hexagon, bow-front, corner-pentagon and L-shaped aquariums.
For a standard rectangular tank, volume is simply length times width times height in inches, divided by 231 (the number of cubic inches in a US gallon). Keep in mind that your true water volume is always a little less than the tank’s rated size once you account for substrate, rock and the gap below the rim — so use your measured water level for the most accurate dosing and stocking numbers.
Formula: Gallons = (L × W × H in inches) ÷ 231 • Cylinder: (π × r² × H) ÷ 231
Estimate your tank volume
Rectangle
Cube
Hexagon
Octagon
Flat Back Hexagon
Cylinder
Half Cylinder
Corner Cylinder
Elliptical
Corner Pentagon
L-Shaped
Corner Prism
Standard aquarium dimensions and usable gallons
Use this fish tank size chart when you know the common tank size but cannot measure the inside water area yet. Dimensions are typical exterior measurements and can vary by manufacturer. The usable-water range subtracts roughly 10 to 15 percent for the space below the rim, substrate, rock and equipment, so measure your own water line before dosing.
| Common tank size | Typical dimensions (L × W × H) | Estimated usable water |
|---|---|---|
| 10 gallon | 20″ × 10″ × 12″ | 8.5–9 gallons |
| 15 gallon | 24″ × 12″ × 12″ | 12.5–13.5 gallons |
| 20 gallon High | 24″ × 12″ × 16″ | 17–18 gallons |
| 20 gallon Long | 30″ × 12″ × 12″ | 17–18 gallons |
| 29 gallon | 30″ × 12″ × 18″ | 25–26 gallons |
| 40 gallon Breeder | 36″ × 18″ × 16″ | 34–36 gallons |
| 55 gallon | 48″ × 13″ × 21″ | 47–50 gallons |
| 75 gallon | 48″ × 18″ × 21″ | 64–68 gallons |
| 90 gallon | 48″ × 18″ × 24″ | 77–81 gallons |
| 125 gallon | 72″ × 18″ × 21″ | 106–113 gallons |
| 180 gallon | 72″ × 24″ × 25″ | 153–162 gallons |
Aquarium volume examples by tank shape
Use inside measurements to the actual water line. Each example is a gross-volume estimate before subtracting displacement from substrate, rock and equipment.
Rectangular aquarium
A tank measuring 36 by 18 by 16 inches holds about 44.9 gallons: (36 × 18 × 16) ÷ 231 = 44.9.
Cylinder aquarium
A round tank with a 10-inch radius and 24-inch water height holds about 32.6 gallons: (π × 10² × 24) ÷ 231 = 32.6.
Half-cylinder aquarium
Using the same 10-inch radius and 24-inch height, a half-cylinder holds about 16.3 gallons because the full cylinder volume is divided by two.
Corner prism aquarium
A triangular corner tank with two 24-inch sides and a 20-inch water height holds about 24.9 gallons: (24 × 24 × 20 ÷ 2) ÷ 231 = 24.9.
Elliptical aquarium
An elliptical tank with radii of 12 and 8 inches and an 18-inch water height holds about 23.5 gallons: (π × 12 × 8 × 18) ÷ 231 = 23.5.
Frequently Asked Questions
How do I calculate the volume of my aquarium?
Multiply the length, width and height of the water area in inches, then divide by 231 to get US gallons. For example, a 24 by 12 by 16 inch tank is 4,608 cubic inches divided by 231, or about 20 gallons. Cylindrical tanks use pi times the radius squared times the height, divided by 231. This calculator applies the right formula for each of a dozen tank shapes automatically.
How can I tell how many gallons my fish tank is?
Measure the inside length, width and water height rather than relying only on the size printed on the tank. Select the closest tank shape, enter those measurements and the calculator will return gallons. If you cannot measure the tank, compare its exterior dimensions with the standard aquarium size chart below for a planning estimate.
Why is my actual water volume less than the tank’s rated size?
A tank rated at 75 gallons rarely holds 75 gallons of water. Substrate, live rock, the space below the rim and equipment displacement all reduce the real water volume, often by 10 to 20 percent. For accurate dosing and salt mixing, base your calculations on measured water volume rather than the label on the box.
How do I convert aquarium liters to gallons?
Divide liters by 3.785 to get US gallons, or multiply gallons by 3.785 to get liters. This calculator switches between imperial and metric with a single toggle, so you can work in whichever units you prefer.
Does this calculator work for round, bow-front and corner aquariums?
Yes. Choose from rectangle, cube, cylinder, half-cylinder, corner cylinder, elliptical, hexagon, flat-back hexagon, octagon, corner pentagon, corner prism and L-shaped tanks. Measure the water area for the selected shape, not decorative trim or an external stand.
Should I use gross volume or actual water volume for dosing?
Use estimated actual water volume for salt mixing, medication and reef dosing. Gross volume is useful for comparing tank sizes, but rock, sand, equipment and the space above the water line commonly reduce operating water volume by 10 to 20 percent.
