How many gallons of water a rectangular tank holds, calculated directly from its inside dimensions.
How it works
Multiplying length, width, and height in inches gives the volume in cubic inches; dividing by 231 (the exact cubic-inch definition of a US gallon) gives gallons.
What this does not include
This does not include the substrate, decorations, or equipment inside the tank, which displace some of that water volume — the actual water needed is typically somewhat less than this full-tank figure.
How to use this calculator
- Enter the tank’s inside length, width, and height in inches.
A worked example
A tank 24 × 12 × 12 inches: volume = (24×12×12) ÷ 231 = 14.961 gallons (231 cubic inches per US gallon).
A tank 36 × 18 × 18 inches: 50.4935 gallons.
What the variables mean
| Variable | Meaning |
|---|---|
| Length, width, height | Interior tank dimensions, in inches |
Edge cases worth knowing
This calculates the tank’s geometric capacity, not the actual water volume after setup. Substrate, decorations, and the water line staying below the rim all reduce the real usable volume below this figure.
A dimension of zero makes the volume zero, a degenerate but valid result — the calculator only declines when a dimension is genuinely missing.
Frequently asked questions
Why use inside dimensions rather than outside dimensions?
The glass or acrylic walls take up some space, so outside dimensions would overstate the actual water capacity.
Why does substrate and decor reduce the actual water needed?
Gravel, rocks, and decorations physically displace water, meaning less water is needed to fill the tank to the same level than the full-tank volume alone suggests.
What’s this most useful for?
Sizing a filter, heater, or medication dose, many of which are rated or dosed by tank gallons.
