Aquarium Volume
Calculator

Multiply the tank's interior length, width, and water height (not the glass height) to get volume, since standard tank sizes like "20 gallon" are rounded nominal figures based on external dimensions, not the exact water capacity once substrate and decorations are added.

Gravel, sand, rocks, and driftwood all displace water, and a couple inches of substrate across a tank's footprint can reduce usable volume by several percent, which matters for accurately dosing medication or fertilizer relative to actual water present.

Bioload capacity, medication dosing, and fertilizer or CO2 dosing in planted tanks are all calculated per gallon or liter of water, so an inflated volume estimate leads to underdosing medication (ineffective treatment) or overstocking fish beyond what the filtration and oxygen levels can support.

Water height matters, since tanks are never filled completely to the rim — typically left an inch or two below the top edge to prevent splashing and allow room for equipment like filters and heaters, so using glass height overstates true volume.

A bow-front tank holds more water than a flat-front rectangular tank of the same nominal length and width because the curve adds volume, so rectangular math slightly underestimates it — most bow-front tanks list a corrected gallon rating for exactly this reason.