Aquarium Volume Calculator
Knowing the exact volume of your aquarium is the starting point for almost every decision in fishkeeping. Medication dosing, dechlorinator quantities, filter sizing, stocking limits, and weekly water change volumes all depend on how many litres or gallons your tank actually holds — not the nominal size printed on the box or listed by the manufacturer. A tank sold as a 55 gallon aquarium rarely holds exactly 55 gallons once you account for glass thickness, substrate depth, headspace at the top, and the displacement from decorations and equipment. The difference between the nominal size and the real usable water volume is often 15 to 25 percent, which matters significantly when you are calculating medication doses or setting up an automatic top-off system.
This aquarium tank volume calculator supports seven common tank shapes including rectangular, bow front, cylinder, sphere, hexagon, corner pentagon, and L-shape. You can enter dimensions in either inches or centimetres, and every result displays simultaneously in US gallons, UK gallons, and litres so there is no need for manual conversion regardless of where you are in the world. The glass thickness dropdown covers all seven standard panel thicknesses from 5 mm nano tanks through to 19 mm very large or commercial aquariums, subtracting the correct amount from all relevant dimensions to give you true interior volume rather than outer dimensions. Deduction sliders for substrate depth, headspace, decorations, and equipment displacement give you the usable water volume — the figure you should use for medication dosing, bioload calculations, and stocking limits.
Use the ProHobby™ Aquarium Water Volume Calculator to accurately determine your tank’s capacity. Calculate accurate aquarium volume using internal dimensions, substrate depth adjustments, and real-world water capacity . With 7 different tank shapes and 13 size presets, get results in both litres and gallons.
Built for real aquariums — not just length × width × height formulas. Free to use, no signup required.
Aquarium volume calculations are straightforward for rectangular tanks but become more involved for unusual shapes. A cylinder holds approximately 78 percent of the volume of a rectangle with the same outer dimensions. A regular hexagon holds around 65 percent of its bounding rectangle. A bow front tank holds roughly 87 percent. These differences matter because many hobbyists simply multiply the length by the width by the height of any tank shape and get a significantly wrong answer. The calculator handles the geometry of each shape automatically — you just measure the outside of the tank and select the glass thickness.
For saltwater and reef tanks, accurate volume becomes even more critical because calcium, alkalinity, and trace element dosing is calculated directly from system volume. Including your sump volume in the calculation gives you true system volume, which is the figure reef keepers need for two-part dosing, kalkwasser, and calcium reactor output calculations. The filled tank weight display — in both kilograms and pounds — is a practical reminder that a 200 litre aquarium weighs over 200 kg when full, before accounting for the tank itself, stand, substrate, and decorations. Always verify your floor can support the total load before filling any tank over 150 litres.
The 25 percent water change volume shown in the results panel tells you exactly how much water to prepare in litres, US gallons, and UK gallons for your weekly maintenance. Bookmark this page and use the preset selector to load your tank dimensions instantly whenever you need to calculate a medication dose or plan a water change.