Tank volume
Rectangular, bowfront, cylindrical, hex, and custom shapes. Reports gallons (US and imperial) and liters.
How this works
Glass thickness is ignored. The math uses outside dimensions, which slightly overstates the water volume. For most stocking decisions, that doesn't matter. For dose calculations, subtract maybe 5 percent if you want to be conservative.
The bowfront calculation treats the front bow as a circular arc. If your tank has an unusual curve (some custom builds use a parabolic front), the result will be off by a few percent.
Why this matters
Tank volume drives stocking, heater wattage, water change volume, and dose amounts for medications and fertilizers. If you only know your tank by its nominal label ("a 75 gallon"), you should probably measure it. Manufacturer labels round generously.