Method: For a full circular pipe: Q = A × V and A = π × D² ÷ 4. Therefore: D = √(4Q ÷ πV). Flow is converted internally to m³/s and velocity to m/s before calculating the required internal diameter. A target velocity of around 1.5 m/s is a practical starting point for many water systems. Velocities above 2 m/s are not automatically unacceptable, but pressure loss, noise, erosion and water-hammer effects should be checked. Actual pipe selection should use a commercially available pipe with an internal diameter at least equal to the calculated requirement.