Tank volume calculator
Work out the total volume of your aquarium and a realistic estimate of how much water it actually holds.
Updated 5 June 2026
Results
The estimated water volume accounts for typical substrate depth (around 5 cm) and the gap between the water line and the rim (around 3 cm). Your actual water volume will vary with your specific setup.
How the calculations work
The total volume is a straightforward geometric calculation based on the shape you've selected. Rectangular and cube tanks use length × width × height. Bow-front tanks add an approximation of the curved front (treated as a half-cylinder along the length of the tank). Cylindrical tanks use π × radius² × height, where the radius is half the diameter you enter.
The estimated water volume is the total volume minus typical losses: roughly 5 cm of substrate at the bottom, and around 3 cm of air gap at the top between the water surface and the tank rim. These are sensible defaults for community freshwater tanks. Tanks with deep planted substrate, large decorations, or unusually high water lines will deviate from the estimate.
Why this matters
Tank volume is the single most useful number in fishkeeping. It determines appropriate stocking, heater wattage, filter flow rate, and dosing for medications, fertilisers, and dechlorinator. Manufacturer-stated tank sizes are often rounded or marketing-friendly — actual water volume is what matters for fish welfare and equipment sizing.
Once you know your water volume, you can choose appropriate species for your tank size, calculate water change volumes accurately using our water changes guide, and match equipment to actual capacity rather than stated capacity.