Aquarium Tank Volume Calculator
Enter your tank’s dimensions to get its gross and estimated usable water volume.
Gross volume: 10.4 gal (39.3 L)
Usable volume (est.): 9.4 gal (35.4 L)
Usable estimate assumes roughly 10% of gross volume is displaced by substrate, decor, and equipment, and that the tank isn’t filled to the rim — see the FAQ below.
Use the result in gallons with the stocking calculator to estimate a stocking level for this tank.
Frequently asked questions
- How is gross volume calculated?
- Exact geometry: length x width x height for a rectangular tank, or pi x radius² x height for a cylinder. Cubic inches convert to US gallons by dividing by 231 (the exact legal definition of a US gallon), and gallons convert to liters by multiplying by 3.785411784 (the exact US-gallon-to-liter conversion).
- Why is my tank's "usable" volume less than its gross volume?
- A tank is never filled to the rim, and substrate, decor, a heater, and a filter's intake and media all displace water beyond the empty-tank geometry. This calculator estimates usable volume as 90% of gross volume, a commonly cited fishkeeping rule of thumb — not a physical constant. A tank with deep substrate or large decor has meaningfully less usable water than this estimate; treat it as a planning aid, not an exact figure.
- Why doesn't a "10-gallon" tank compute as exactly 10 gallons?
- Manufacturer size names are marketing round numbers, not the tank's exact glass volume. A standard 10-gallon tank's real dimensions (20 x 10 x 12 inches) come out to about 10.4 gross gallons by the geometry above — and less once you subtract substrate and decor. This is normal and applies to most rated tank sizes, not just 10-gallon tanks.
- Does this support bowfront, hexagon, or other irregular shapes?
- Not yet — only rectangular and cylindrical tanks have exact, unambiguous geometry formulas. For a bowfront, hexagon, or other irregular shape, check the manufacturer's own published volume rather than estimating from this calculator.