How to size water pipe?

Pipes which are mainly made of copper, steel and plastic are used to supply water to a house for daily use. The piping system needs to be efficient for long life span, robust, safe, and durable.

Flow rate

To size a water pipe the desired flow rate need to be estimated. 

Fluid velocity

The fluid flow velocity should not exceed a certain value to avoid noise and damaged and tear of pipes or fittings. The table below show the maximum fluid velocities for some water application in a house. Pipe anfd fitting must be sized so that the flow rates at the flow rates in the table above are respected. As the BS 6700, the flow velocities must not exceed 3 m/s, BS EN 805 recommends flow velocities in the range 0.5 m/s to 2m/s with maximum velocity of 3.5 m/s in certain cases.

Pressure or Head loss, HL

For a given average velocity of fluid, there is head loss due to friction along a given length of pipe. The roughness (mm) for different pipe materials is given below.

Pipe surface roughness, ε

Pipe roughness is an important factor to be taken in consideration for pipe sizing.

K value

For example: -


Water flowing through a PVC pipe (1-inch diameter) at 20 ⁰C at 0.5 m/s.


ρ = 1000 kg/m 3(water)  

v = 0.5 m/s

D = 0.0254 m (1 inch)

µ = 0.001 Pa.s (water @ 20 ⁰C)

ε = 0.0015


Water Pipe sizing

The correct pipe sizing must be ensured while designing a plumbing system for adequate flow rates at utility points.


Oversized system:

  • Additional installation cost
  • Delays at water outlets
  • increased heat loss in hot water piping

Undersized system:

  • Less or even no delivery at water outlets
  • Less pressure and temperature at water outlets
  • Increase in noise level
Equivalent pipe length

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