Assumptions and misconceptions about aquarium volume
The math behind this calculator is exact for a rectangular box, but the number people actually need — the usable water volume of a decorated, stocked tank — involves a few assumptions worth stating plainly.
What the calculator assumes
It assumes a straight-sided rectangular prism measured on the inside of the glass, a uniform fill height across the whole footprint, and freshwater at roughly room temperature for the weight estimate. Bowfront, hexagon, corner, and cylinder tanks are not rectangular, so this formula will not match their true volume — a cylindrical tank needs π × radius² × height instead.
Misconceptions to avoid
"Filling to the rim gives me the most water and the most stocking room."
Filling past 90% or so risks overflow once a hood, filter return, heater, and decorations are added, and leaves no headroom for evaporation between top-offs. Stocking capacity is also about surface area and filtration, not just raw water volume.
"The calculated volume is exactly what I'll have once the tank is set up."
Substrate, rock, driftwood, and internal equipment all displace water. A few inches of gravel or a large piece of hardscape can reduce the real water volume by 5-15% compared to the empty-tank calculation.
"A tank's labeled gallon size is its precise volume."
Labeled sizes (10 gallon, 55 gallon, and so on) are nominal, rounded figures used for marketing and standard-size accessories. The measured interior volume is usually a little less.
Where this differs from other shapes
This tool only models rectangular tanks. For a cylindrical tank, volume is π × radius² × height; for a bowfront or hexagon tank, treat the rectangular result as an upper-bound estimate, since the curved or angled front reduces the true volume below what length × width × height would suggest.
Precision limits
This is a planning estimate, not a substitute for a graduated container when precise dosing matters — for critical medication dosing in a small tank, measuring the actual water volume directly is more reliable than any geometric calculation.
Understanding these limits keeps the number honest: it is the volume of an empty rectangular box at your chosen fill level, a solid starting point that real-world setup will adjust slightly.