Aquarium Volume Calculator
Enter your tank's dimensions and see how much water it holds — instantly, in liters and US gallons, for rectangular, bow-front, cylinder and corner tanks.
Volume
200 L
≈ 52.8 US gal
Formula: length × width × height, converted to liters (1 L = 1000 cm³).
This is the gross, brim-full volume. Substrate, decor and the fill line usually leave the real water volume 10–15% lower.
How we calculate
Every tank shape here reduces to base area × height. A rectangular tank's base is length × width. A cylinder's base is π × radius². A corner tank is modeled as a quarter cylinder whose two straight sides meet at 90°. A bow-front adds a curved segment to a rectangular base; we approximate that segment as ⅔ × length × bow depth (the parabolic-segment rule), which stays within about 2% of a true circular segment for typical bow depths.
The result is the gross, brim-full volume of the glass box — the number manufacturers print on the label. The water your fish actually live in is less: substrate takes several liters, hardscape and equipment displace more, and nobody fills to the rim. Plan on 10–15% below the gross figure. Our stocking calculator already applies a usable-volume correction on top of the gross volume, so you can pass this number straight into it.
All math runs in metric and is converted for display only: 1 inch = 2.54 cm, 1 US gallon = 3.785 liters. Note that imperial (UK) gallons are larger — 4.546 liters — so a '20 gallon' UK tank holds about 20% more water than a US one.
Frequently asked questions
- How do I measure my aquarium for this calculator?
- Measure the outer glass edges in a straight line: length along the front, width front-to-back, height bottom to rim. For a bow-front, take the width twice — at the side panel and at the deepest point of the curve in the middle. Measuring outside includes the glass thickness, so the true inner volume is slightly lower — one more reason to treat the result as a gross figure.
- How many liters are in a gallon?
- One US gallon is 3.785 liters; one imperial (UK) gallon is 4.546 liters. This calculator shows US gallons. If your tank was sold in UK gallons, multiply by 1.2 to get the US-gallon equivalent.
- Why does my tank hold less water than the calculator says?
- The calculator returns the brim-full volume of the empty box. Substrate, rocks, wood, an internal filter and the gap you leave below the rim all subtract from it — typically 10–15% in a normally decorated tank, and more in heavily hardscaped aquascapes.
- What tank volume do my fish actually need?
- That depends on each species' adult size, bioload and schooling needs, not on volume alone. Once you know your volume, use the stocking calculator to test your exact combination of fish against it — it flags overstocking, undersized tanks and incompatible pairs.