Aquarium Tank Volume Calculator
Enter your tank’s dimensions to get its volume in litres and US gallons. The calculator also estimates real water volume — what’s actually in the tank after you account for the fill line and substrate — which is the number you should use for dosing and stocking.
Calculate tank volume
1 US gallon = 231 cubic inches ≈ 3.785 L. Real volume assumes water filled to ~5 cm (2 in) below the rim minus substrate.
How the calculation works
Volume is simple geometry. For a rectangular tank it is length × width × height; for a cylinder it is πr² × height. The calculator converts cubic centimetres to litres (1,000 cm³ = 1 L) and to US gallons using the statutory definition of exactly 231 cubic inches per gallon — the same conversion used by every aquarium manufacturer.
Bowfront tanks have no exact closed formula, so we approximate the bow as half of an ellipse: the bulge adds roughly π/4 × length × half the bow depth to the footprint. For typical bowfronts this lands within a few percent of the manufacturer’s figure.
Nominal vs. real volume — why it matters
A “20 gallon” tank almost never holds 20 gallons of water. Three things eat into the nominal figure: the glass itself (external vs. internal dimensions), the air gap you leave at the top, and everything you put inside — substrate, rock, wood. A realistic planted 20-gallon often holds 15–17 gallons of actual water.
This matters most when you dose anything: medications, fertilisers, and water conditioners are all dosed per real litre. Overdosing medication because you used the sticker volume is one of the most common beginner mistakes in the hobby.
Common mistakes
- Measuring external dimensions and treating them as internal. On a 12 mm glass tank this alone overstates volume by ~4–5%.
- Using nominal volume for dosing. Use the “real water volume” figure, or better, note exactly how many litres you add when you first fill the tank.
- Forgetting displacement. A large piece of hardscape can displace several litres. If in doubt, fill with a measured bucket once.
- Mixing up US and imperial gallons. A UK (imperial) gallon is 4.546 L, about 20% bigger than a US gallon. This calculator uses US gallons.
Frequently asked questions
How much does substrate reduce my water volume?
Multiply your footprint by the substrate depth: a 90×45 cm tank with 5 cm of substrate loses about 20 L. The calculator does this automatically if you enter a substrate depth. Note that gravel is porous, so the true loss is around 60–70% of that figure — treating it as 100% keeps your dosing conservative.
Should I use nominal or real volume for stocking calculations?
Real volume. Stocking guidelines, filter turnover, and heater sizing all assume actual water. Feeding the nominal sticker volume into those tools quietly overstates your tank by 15–25%.
My manufacturer says my tank is a different size than this calculator. Why?
Manufacturers usually quote brim-full volume from external dimensions and round to the nearest marketing-friendly size. Measure the inside of the tank and expect the honest number to be smaller.
How do I measure a hexagon or other odd shape?
The practical answer: fill it with a container of known size and count. For a regular hexagon you can also compute footprint = 2.598 × side² and multiply by height, then divide by 1,000 for litres.