Every stocking choice, dose and water change starts with one number: how much water your tank actually holds. This aquarium volume calculator works for any fish tank, freshwater or salt. Measure the glass, pick your shape, and it returns gallons, liters and imperial gallons at once, plus the filled weight so you know your stand can take it. Think of it as a fish tank gallon calculator and a liter converter in one.
Aquarium Volume Calculator
Enter your tank's measurements and get the true volume in gallons and liters. Works for rectangle, bow-front, cylinder, and hexagon tanks.
One US gallon = 231 cubic inches = 3.785 liters. The usable water volume is lower than the raw figure because substrate, rocks, and the gap below the rim displace water. For stocking and dosing, use the usable number.
Why the number on the box is wrong
A “20 gallon” tank is a marketing label rounded from the outside dimensions. Substrate, rocks and the gap you leave below the rim all eat into the real figure. Most tanks hold roughly 10 to 15 percent less water than the label once they are scaped, which is why this tool shows a usable volume alongside the full one. Dose and stock against the usable number, not the sticker.
How to measure your tank
Measure the inside glass in a straight line, not the outside. For a rectangular tank you need length, width and height. For a bow front, add the depth of the bulge at its widest point. For a cylinder or hexagon, measure across the flat and the height. Enter the figures in inches or centimeters and switch units with the toggle.
From dimensions to gallons
Enter length, width and height and this aquarium dimensions calculator converts them into water volume, so you never have to hunt for the manual. It works as an aquarium water volume calculator behind the scenes: length by width by height, divided by 231 for US gallons or by 1,000 for liters, with the shape math for bow fronts, cylinders and hexagons handled automatically.
One calculator for aquariums and fish tanks
Aquarium or fish tank, the water inside is the same, so this doubles as a fish tank volume calculator and a fish tank capacity calculator. Whether you searched for an aquarium size calculator or just want to know how many gallons your tank holds, enter the same three measurements and read off the result.
Gallons, liters and water weight
One US gallon of freshwater weighs about 8.34 pounds; one liter weighs one kilogram. As an aquarium gallon calculator it shows US and imperial gallons side by side, which matters when a dosing product is labelled for one but not the other. A filled 40 gallon tank plus glass, gravel and rock pushes well past 400 pounds, so check the weight line against what your furniture and floor can hold before you fill.
Put the number to work
Once you know your true volume, size the rest of the system around it. Use our aquarium stocking calculator to see how many fish that water can support, match a filter with our guide to filter types, and check the wattage on our aquarium heater size calculator. Ready to fill it? Follow the full fish tank setup guide. For every tool in one place, visit the aquarium calculators hub.
Frequently asked questions
How do I calculate the volume of my fish tank?
Multiply length by width by height in inches and divide by 231 for US gallons, or measure in centimeters, multiply the three and divide by 1000 for liters. This calculator does it for rectangular, bow front, cylinder and hexagon tanks automatically.
How many gallons is my aquarium or fish tank?
The same math answers both: length times width times height in inches, divided by 231. A standard 24 by 12 by 16 inch tank works out to about 20 gallons. Enter your own measurements above and the calculator tells you how many gallons your tank holds instantly.
How much water is really in a “10 gallon” tank?
Closer to 8 or 9 gallons once you subtract substrate and the air gap below the rim. Use the usable volume line for dosing and stocking decisions.
How much does a filled aquarium weigh?
Roughly 10 pounds per gallon of freshwater once you add glass, substrate and hardscape. A 55 gallon tank lands around 600 pounds filled.