Pipe Sizing & Flow Calculator

The formula

Velocity is flow divided by the pipe's internal cross-sectional area: v = Q ÷ A, where A = π × (ID ÷ 2)².

Flow in L/min is converted to m³/s by dividing by 60,000, and the internal diameter in mm is converted to metres. For example 18 L/min through 15mm copper (13.6mm ID): 0.0003 ÷ (π × 0.0068²) ≈ 2.07 m/s — too fast, so 22mm is recommended at ≈0.94 m/s.

UK velocity limits

BS EN 806 and CIBSE guidance recommend keeping water velocity in domestic pipework to roughly 1.5 m/s in most situations. Above this, water hammer, whistling at fittings and general flow noise become noticeable — especially in bedrooms and where pipes run through joists. Higher velocities also increase friction losses (pressure drop) and can erode copper at elbows over time.

Use 1.0 m/s for noise-sensitive runs and heating circuits, and treat 2.0 m/s as an absolute ceiling for short runs in plant rooms. Always check available pressure and pressure drop across the whole run too — this tool only checks velocity.

Plastic pipe has a thicker wall than copper of the same nominal size, so its internal bore is smaller and velocity is higher for the same flow. PEX internal diameters vary by manufacturer and whether a pipe insert is used; values here are typical.

Standard UK pipe internal diameters