How to Calculate Water Tank Capacity in Litres

Capacity in litres falls straight out of dimensions in centimetres, because 1,000 cm³ = 1 litre exactly.

Cylindrical (most overhead tanks): litres = π × (D/2)² × H ÷ 1000, with D and H in cm. A 110 cm diameter tank 110 cm tall: 3.1416 × 55² × 110 ÷ 1000 ≈ 1,045 L — the familiar "1000 litre" class.

Rectangular (sumps): litres = L × W × H ÷ 1000 in cm. A 2 m × 1.5 m × 1.5 m sump is 200 × 150 × 150 ÷ 1000 = 4,500 L.

Partly filled tanks are where mental math fails for round shapes — a horizontal cylinder at half depth is exactly half volume, but at a quarter depth holds only ~19.6%. The litres calculator handles every shape at any fill level with exact geometry, and shows the maths on the methodology page.