How much water does your tank actually hold once the substrate, rocks and equipment are in? Rectangular length × width × height doesn’t give you the answer — it gives you the theoretical maximum. This calculator gives you the realistic figure.

Why the number matters

Every stocking rule, medication dose, water-change calculation and CO2 bubble-rate target is expressed per litre or per gallon. Getting the volume right is the whole basis of the rest of the hobby.

The 12% deduction we apply by default is a UK aquarist standard. A tank filled to the brim with only water rarely happens outside a display room. Substrate typically takes 6–10% of internal volume, rocks and wood another 2–4%, filter intakes and heaters the rest. Uncheck the box if you want the theoretical maximum.

Notes on the shapes

• Rectangular — most standard glass tanks. Length × width × height in cm ÷ 1000 = litres.
• Bow-front — front glass curves outward. We apply an ~8% expansion factor over the equivalent rectangular volume, which matches typical UK bow-front tanks (Aquael, Fluval, Juwel bow-front models).
• Cube — one side length applied to all three axes. Enter your side length in Length.
• Cylinder — enter the diameter as "Length" and the height. Uses π r² × h.
• Hexagonal — enter the side length (edge-to-edge across the flat, not corner-to-corner). Applies the regular hexagon area formula × height.

The stocking rule you'll see (and why it's wrong)

The "one inch of fish per gallon" rule is the most-repeated stocking rule online. It's also an oversimplification that gets fish killed. A 15cm plecostomus produces vastly more waste than five 3cm neons of the same combined length. Adult tank size, filter turnover, water changes and swimming habits all matter more than length-per-gallon.

Use this calculator to get your volume right, then use the volume as an input to species-specific stocking guides. See our beginner stocking guide for realistic UK-community suggestions by tank size.

— Jaqline Smalling, FishForever