Methodology

This page explains exactly how Fish Tank Volume Calculator produces every result, so you can trust the numbers and reproduce them yourself. Every calculator uses a published geometric or physical formula, not an estimate.

Volume formulas

Tank volume is calculated from the exact geometry of each shape:

• Rectangular and cube: length times width times height.
• Cylinder: pi times radius squared times height.
• Half and quarter cylinder: the full cylinder volume divided by 2 or 4.
• Bow front: the rectangular volume plus a circular-segment correction.
• Hexagon, octagon and pentagon: the regular-polygon area times height.
• Triangle: half of length times width times height.

Unit conversions

We use the internationally accepted conversion factors. One US gallon is 3.785 litres (231 cubic inches) and one UK gallon is 4.546 litres (277.4 cubic inches). One litre equals 1,000 cubic centimetres. All inputs are converted to a common unit before the volume is computed, then converted to your chosen output unit.

Weight

Fresh water weighs approximately 1 kilogram per litre (about 8.34 pounds per US gallon). The weight calculator multiplies the water volume by this density. The total filled weight of a tank also includes the glass, stand, substrate and rocks, which we note alongside the water weight.

Substrate

Substrate volume is tank length times width times depth. Weight is the volume multiplied by the material density: approximately 1.5 kilograms per litre for gravel, 1.6 for sand and 0.9 for aqua soil. Recommended depths are 2.5 to 5 cm for fish-only tanks and 5 to 7.5 cm for planted tanks.

Stocking

The stocking estimate uses the widely cited guideline of 1 cm of adult fish per litre of water, equivalent to roughly 1 inch of fish per US gallon. We present it as a starting point and clearly state that filtration, species, adult size and territory all affect a safe stocking level.

Glass thickness

Glass thickness recommendations follow standard safe-thickness charts, which use a safety factor of about 3.8 for regular float glass and about 7.6 for tempered glass. Thickness rises with water height, because pressure increases with depth, and with the unsupported length of the panel. For tall or large tanks we recommend professional verification.

Accuracy

Results reflect the full internal capacity for the dimensions you enter. Real water volume is typically 10 to 15 percent lower once substrate, rocks, décor and the surface gap are accounted for, which we explain on the relevant pages. Measure the inside of the glass for the most accurate input.

Last reviewed: June 2026. See our editorial standards for how we keep these methods accurate.