Aquarium Volume
This free aquarium volume calculator estimates how many gallons and liters a rectangular or cylindrical tank holds, from its dimensions in inches. Useful for stocking, medication dosing, and figuring out what a tank actually holds versus its store-sold nominal size. Nothing you enter is sent anywhere or stored.
How to Use It
- Select your tank shape: Rectangular or Cylinder.
- Enter the tank's dimensions in inches.
- The estimated gallons and liters update instantly, no submit button.
How It Works
This calculator finds the tank's volume in cubic inches from its shape and dimensions, then converts that figure to gallons (dividing by 231, the number of cubic inches in a US gallon) and to liters (multiplying by 0.016387064, the number of liters in a cubic inch). The conversion itself never changes, only how the volume in cubic inches is found in the first place depends on the tank's shape.
Formulas
rectangular: volume = length × width × height
cylinder: volume = pi × (diameter ÷ 2)² × height
gallons = volume ÷ 231, liters = volume × 0.016387064
A worked example: a standard "20 gallon long" tank measures 30 by 12 by 12 inches, giving 4,320 cubic inches, which works out to about 18.7 gallons (70.8 liters), close to its nominal 20 gallon name but not exact, since that name is a rounded marketing size rather than the tank's precise geometric volume.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
Why doesn't a "20 gallon" tank calculate to exactly 20 gallons?
Store-sold tank sizes like "20 gallon" or "10 gallon" are rounded marketing names, not the tank's exact geometric volume. Manufacturers build tanks from standard glass panel sizes, so the actual length, width, and height rarely multiply out to a perfectly round gallon figure. A "20 gallon long" tank, for example, is typically 30 by 12 by 12 inches, which calculates to about 18.7 gallons, close to 20 but not exact.
Why is the actual water volume less than the calculated tank volume?
This calculator finds the volume of the empty glass box or cylinder. Once substrate, decorations, equipment, and the gap you leave below the rim (so water doesn't overflow when fish or decorations displace it) are accounted for, the water you actually add is typically 80 to 90 percent of the calculated figure. That range is a rule of thumb from the fishkeeping hobby, not a precise calculation, since it depends heavily on how much substrate and decor a given tank has.
When should I use liters instead of gallons?
Liters are the standard unit outside the US, and many international fish and plant care guides, along with some medication dosing instructions, are written in milliliters per liter rather than per gallon. This calculator shows both, so you can use whichever unit matches the product or care guide you are working from.
Does tank shape change how the volume is calculated?
Yes. A rectangular tank's volume is simply length times width times height. A cylindrical tank uses the area of a circle (pi times the radius squared) times height instead, since a round cross-section holds less water than a square one with the same outer width.
Can you give a real-world example?
A standard "20 gallon long" tank, 30 by 12 by 12 inches, holds about 18.7 gallons (70.8 liters). A cylindrical tank 12 inches across and 16 inches tall holds about 7.8 gallons (29.7 liters), noticeably less than a rectangular tank with the same width and height, since the round shape holds less water for the same footprint.