How to use the aquarium volume and stocking calculator
- Choose a rectangular or cylinder tank, then enter the inside measurements in inches. Measure inside the glass, not the outer frame.
- Enter how far the water sits below the top of the tank, and your estimate for the space taken by gravel, rocks and decor.
- Read the volume of the full tank, the volume at the water line, the net water volume in gallons and liters and the weight of the water.
- Use the net volume, not the size on the label, for medication doses and water conditioner.
Formula
Rectangular: gallons = length x width x water height / 231
Cylinder: gallons = pi x (diameter / 2)^2 x water height / 231
Net water = gallons at the water line x (1 - displacement)
1 US gallon = 231 cubic inches = 3.785 liters; water weighs about 8.34 lb per gallon
A US gallon is exactly 231 cubic inches. The weight figure is for fresh water at room temperature and ignores the glass, stand, gravel and decor.
Worked example
A tank measures 48 x 13 x 20 inches inside. Filled to the rim it holds 48 x 13 x 20 / 231 = 54.0 gallons. With a 1 inch gap the water height is 19 inches, giving 48 x 13 x 19 / 231 = 51.3 gallons.
Gravel and decor take about 10 percent, so the net water is 51.3 x 0.9 = 46.2 gallons, which is 174.9 liters and weighs about 385 lb.
A round tank 18 inches across and 20 inches tall, filled to the top, holds 3.1416 x 81 x 20 / 231 = 22.0 gallons.
Label size versus real volume
A tank sold as a 55 gallon is rated to its brim, but you will never fill it to the brim. The water line, the thickness of the glass and the substrate all reduce what is really in the tank. The difference can be 10 to 20 percent, which matters when you dose a treatment that is calculated per gallon.
Measure the inside length, width and height with a ruler or tape. Glass thickness on large tanks can be a quarter inch or more, so using outer measurements overstates the volume.
A note on stocking
People often ask how many fish a tank holds. Volume is only one part of the answer, and the old rule of one inch of fish per gallon is a rough starting guess for small, slim species.
- Large fish, messy fish and fish that grow quickly need much more space than the rule suggests.
- Filtration, water changes and plants decide how much waste the tank can handle.
- Many species need a minimum tank length for swimming and territory, not just volume.
- Research every species and add fish slowly so the tank can build a healthy colony of beneficial bacteria.
Assumptions and limits
- The tank is a plain rectangular box or a straight cylinder. Bow fronts, hexagons and corner tanks are not modeled.
- The displacement percentage is your own estimate. It is not measured.
- The weight shown is water only, using 8.34 lb per US gallon, and does not include the tank, stand, substrate or equipment.
- This tool gives volume and does not advise on fish stocking or medication.
Frequently asked questions
How do I calculate the gallons in a fish tank?
Multiply the inside length, width and water height in inches, then divide by 231. That gives US gallons. This calculator also handles a cylinder, the water line and the space taken by gravel and decor.
Why is my 55 gallon tank holding less than 55 gallons?
The rated size assumes a tank filled to the very top with nothing inside. A real tank has a gap at the top, thick glass and gravel and decor that displace water, so the water you add is usually 10 to 20 percent less.
How much does a filled aquarium weigh?
Water alone is about 8.34 pounds per US gallon, so a 46 gallon fill is about 385 pounds. Add the glass, stand, gravel, rock and equipment, and check that your floor and stand can carry the total.
How many liters are in a US gallon?
One US gallon is 3.785 liters. The calculator shows both units for the net water volume so you can use either on a dosing label.