The Hazen-Williams equation for calculating the pressure drop due to friction for a given pipe diameter and flow rate is as follow

### The equation in SI units

where

P = frictional pressire drop, kPa/m
Q = flow rate, m^3/h
D = pipe inside diameter, mm
C = Hazen-Williams C factor, dimensionaless
In general an average value of C=100 is used for most applications.

The Hazen-Williams C factor depends on the type of the pipe material and the internal condition of the pipe.
A low value such as C=75 may be used for pipe that is 10 to 15 yeas old

### The equation in U.S. customary units

where

P = pressure loss due to friction, psi per ft of pipe length
Q = flow rate, gal/min
D = pipe inside diameter, in
C = Hazen-Williams roughness coefficient factor, dimensionaless
**Hazen-Williams C Factor Table**
**Pipe material** | **C factor** |

Smooth pipes (all metal) | 130-140 |

Cast iron (old) | 100 |

Cast iron (unlined new) | 120 |

Iron (worn/pitted) | 60-80 |

Polyvinyl chloride | 150 |

Brick | 100 |

Smooth wood | 120 |

Smooth masonry | 120 |

Vitrified clay | 110 |

Plastic | 150 |

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