About the Fish Tank Volume Calculator
This fish tank volume calculator works out how much water your aquarium holds from its inside dimensions. Choose the tank shape — rectangular, cylinder, hexagonal or quarter-circle corner — enter the measurements in inches or centimetres, and get the volume in US gallons and liters, the actual water volume at your fill level, the weight of that water and a rough stocking guide.
Knowing the true volume matters for dosing medications, water conditioners and fertilisers, sizing filters and heaters (often rated per gallon), and checking that a stand or floor can take the weight. Water alone weighs about 8.34 lb per US gallon, so even a 20-gallon tank weighs well over 150 lb once glass, substrate and décor are added.
Measure the inside of the glass if you can; outside measurements slightly overstate the volume. The stocking figure uses the traditional “one inch of adult fish per gallon” rule, which is only a starting point for small, slim community fish — large, messy or territorial species need much more space, and good filtration and water changes matter more than any rule of thumb.
With the default inputs, the tank volume is 18.7 US gal. Change any value above to recalculate instantly.
How to use the fish tank volume calculator
- 1Choose the shape of your tank.
- 2Pick inches or centimetres and measure the inside dimensions.
- 3Enter the height and the level you fill the water to.
- 4Read gallons and liters, then use the water volume for dosing and filter sizing.
Formula and method
Volume is the area of the tank’s footprint multiplied by its height. Base areas: rectangle = length × width; cylinder = π × (diameter ÷ 2)²; regular hexagon = (3√3 ÷ 2) × side²; quarter-circle corner tank = π × radius² ÷ 4. Centimetres are converted to inches first, and 231 cubic inches make one US gallon.
The water volume multiplies the full volume by your fill level, water weight uses 8.34 lb per US gallon, and the stocking guide applies the traditional one-inch-of-fish-per-gallon rule to the water volume.
- area
- Footprint area of the tank in square inches
- 231
- Cubic inches in one US gallon
- 3.785
- Liters in one US gallon
Worked examples
30 × 12 × 12 inch tank (20 gallon long)
30 × 12 × 12 = 4,320 cubic inches, and 4,320 ÷ 231 = 18.7 US gallons (70.8 L). Tanks are sold by outside dimensions, so a “20 gallon long” holds slightly less. At a 90% fill that is 16.8 gallons of water weighing about 140 lb.
Metric 100 × 40 × 50 cm tank filled to the top
100 × 40 × 50 cm = 200,000 cm³, which is exactly 200 liters or about 52.8 US gallons.
18-inch diameter cylinder, 24 inches tall
The base area is π × 9² ≈ 254.5 square inches; times 24 inches is about 6,107 cubic inches, or 26.4 gallons (100 L).
Frequently asked questions
How do I calculate how many gallons my fish tank holds?+
Multiply length × width × height in inches and divide by 231 to get US gallons. For centimetres, multiply the three dimensions and divide by 1,000 to get liters, then divide by 3.785 for gallons.
How much does a fish tank weigh when full?+
Water weighs about 8.34 lb per US gallon (1 kg per liter). Add roughly 1.5–2 lb per gallon for glass, substrate and rock, so a filled 20-gallon tank typically weighs around 200–225 lb.
Is the one inch of fish per gallon rule accurate?+
Only loosely. It works as a rough ceiling for small, slim fish such as tetras and rasboras, but not for bulky, messy or territorial fish like goldfish or cichlids. Filtration, surface area and adult fish size matter more.
Why is my tank’s real volume less than its label?+
Tanks are named by outside dimensions and nominal size. Glass thickness, the gap at the top, substrate and decorations all take up space, so actual water is often 10–20% less than the label.
How much water should I change each week?+
A common routine is 10–25% of the water every week, adjusted to your stocking level and test results. Always treat new tap water with a dechlorinator dosed for the volume you add.