Public Health

Water Pipe Size Calculator

Estimate domestic water pipe size from flow rate and target velocity.

Use this calculator for preliminary domestic cold water and boosted water pipe sizing checks based on a target maximum velocity. It can size from a known design flow rate and select the smallest listed pipe size that keeps velocity at or below the chosen limit.

Inputs

Pipe selection is based on the smallest listed pipe size that keeps calculated velocity at or below the selected maximum. Final domestic water design should also consider pressure drop, residual pressure and outlet requirements.

Result

Recommended Pipe Size
-
Enter values and click calculate.

Method

Pipe size is selected by comparing the design flow rate against the internal flow area of each listed pipe size.

v = Q ÷ A

Internal pipe area is determined from:

A = πd² ÷ 4
  • v = water velocity in pipe (m/s)
  • Q = volumetric flow rate (m³/s)
  • A = internal pipe area (m²)
  • d = internal pipe diameter (m)
  • The smallest listed pipe size with velocity at or below the target is selected.
  • Calculated result = -

Reference

  • This calculator applies a simple velocity-based method for preliminary domestic water pipe sizing.
  • Different pipe materials use different size series and internal diameters, so the recommended size depends on the selected material.
  • Velocity-based selection is only one part of the design process and should be checked alongside pressure loss and outlet performance.
  • Relevant guidance may include BS EN 806, the Water Supply (Water Fittings) Regulations and project-specific water authority criteria.
Design note: This calculator sizes pipework based on velocity only. Final pipe selection should also consider pressure drop, available supply pressure, boosted water systems, outlet performance, fittings losses, noise, manufacturer data and project-specific design criteria.

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