The numbers behind a thriving aquarium, calculated honestly.

For a rectangular aquarium: multiply length × width × height in centimetres, then divide by 1000 for litres. A 91 × 38 × 45 cm tank is 91 × 38 × 45 = 155,610 cm³ → 155.6 litres gross. Multiply by 0.219969 for UK gallons, by 0.264172 for US gallons.

Cylinders use π × r² × h. Spheres use (4/3) × π × r³. Our calculator handles all three and converts between cm, mm, and inches automatically.

Stated capacity is usually the gross internal volume — the box. In practice you'll fill to about 90% of that, because: substrate eats 5–10% depending on depth; rocks, wood, and equipment displace another few percent; and you'll leave a 1.5–3 cm gap below the rim for filter intakes and CO₂ off-gassing.

Use the realistic water fill figure (the big number on the right of our calculator) when dosing treatments. Use the gross figure only when comparing tanks for purchase.

The classic beginner rule is one inch of small adult fish per US gallon (≈ 2.5 cm per 4 litres). It works as a first sanity check and that's all. Real stocking depends on:

• Bioload per species. A 6 cm fancy goldfish produces an order of magnitude more waste than a 6 cm tetra.
• Body shape and activity. A long-bodied, fast-swimming danio needs swimming length far more than a slow-moving betta.
• Schooling needs. Many species (rasboras, tetras, corys) need groups of 6+, which constrains your minimum tank size before stocking math even begins.
• Surface area. Oxygen exchange happens at the water's surface, so a long shallow tank stocks more comfortably than a tall column of identical volume.

On a 30 cm starter tank, no — the difference is well under a litre. On a 200-litre rectangular tank with 12 mm glass, yes: roughly 4–6 litres of "missing" volume that the glass walls are occupying. For dosing purposes that gap is meaningful when you're working with sensitive medications.

Our calculator includes an optional glass-thickness field. We apply it to rectangular tanks only — for cylinders and bowls the geometry is too dependent on whether the base is glass or moulded plastic.

For most species, yes. A bowl has the worst surface-area-to-volume ratio of any tank shape, which limits oxygen exchange. It's also too small for filtration, too unstable in temperature and water chemistry, and too cramped for any fish that needs to swim more than a body-length in a straight line.

We include the sphere shape in our calculator because people ask for it — typically for a betta or a temporary holding bowl during a water change. If you're shopping for a long-term home, look at rectangular tanks of 40 litres or larger.

Counter-intuitively, bigger is more forgiving. A 75–100 litre rectangular tank is a sweet spot: enough water to buffer the inevitable mistakes (overfeeding, late water changes, a stuck heater), enough surface area for healthy gas exchange, and small enough to lift while empty.

Avoid anything under 30 litres for your first tank. Nano tanks are wonderful when you understand the chemistry — and unforgiving when you don't.