Aquarium Volume Calculator
Get your tank's true water volume, not just the number on the box. Pick from 11 real-world aquarium shapes, enter your dimensions, and see gallons, litres, and what that shape actually means for your fish.
How to use this calculator
- Choose your tank's shape from the groups below. A labeled diagram shows exactly what each measurement refers to.
- Measure your tank and enter the values, using either inches or centimetres.
- Read your gross volume instantly — this is your tank's structural size before anything is added.
- Open "Calculate net (usable) water volume" and enter your substrate depth and water line gap for the volume you'll actually be dosing and stocking around.
- Use the four cards below the results for a quick heater size, water change amount, substrate estimate, and betta suitability check.
Heater size
–
rule-of-thumb estimate
25% water change
–
typical weekly amount
Substrate needed
–
at set depth above
Betta suitability
–
5 gal minimum
These figures are estimates based on standard rules of thumb. Actual results vary with glass thickness, rounded corners, decor, and fish species. Always confirm exact medication or salt dosing against the product label.
Why your tank's true volume matters
The gallon number on the box is almost always higher than what your tank actually holds once substrate, decor, and the gap below the rim are accounted for — and medication, salt, and dechlorinator are all dosed per gallon of actual water, not the advertised size.
Choosing a shape
| Shape | Best for | Trade-off |
|---|
Frequently asked questions
Which aquarium shape is best for beginners?
A standard rectangular tank. It's the easiest to filter, aquascape, and stock, and nearly all beginner equipment, from heaters to filters to lids, is designed around it first.
Why is my tank's actual water volume less than the size printed on the box?
Manufacturers round to a nominal size and measure to the rim, but you never fill a tank completely to the rim, and glass thickness, rounded seams, substrate, and decor all take up space the box's number doesn't subtract.
Does substrate really reduce my usable water volume?
Yes. A 2-inch layer of gravel across a standard 20-gallon footprint can displace close to a gallon of water. It's small per tank, but it adds up fast when you're calculating exact medication or salt doses.
How is heater wattage estimated from tank volume?
This tool uses a common rule of thumb of about 4 watts per gallon of net water volume for an average room. Colder rooms or larger temperature gaps may need closer to 5 watts per gallon.