Hazen-Williams equation is a popular approach for the calculation of head loss of liquid in piping systems. In this method, Hazen-Williams coefficient is introduced to the equation, the coefficient eliminates the need for using Reynolds number. The Hazen-Williams coefficient is mainly used to account for the pipe roughness or efficiency.

## Hazen-Williams equation

Different formats for this equation is specified below

### Equation 1 (US Units)

Hazen-Williams equation for head loss is shown below:

Where;

H = frictional head loss, ft

L = length of pipe, ft

D = inside diameter of pipe, ft

Q = flow rate, ft^{3}/s

C = Hazen-Williams C factor or roughness coefficient, dimensionless

### Equation 2 (US Units)

Where;

H = friction loss, ft of water per 1000 ft of pipe

Q = flow rate, gal/min

D = inside diameter of pipe, in

C = Hazen-Williams C coefficient, dimensionless

### Equation 3 (US Units)

Where;

P_{mi} = friction loss, psi per mile of pipe

Q = flow rate, gal/min

D = inside diameter of pipe, in

C = Hazen-Williams C coefficient, dimensionless

### Equation 4 (US Units) – Flow rate

Where;

H = friction loss, ft of water per 1000 ft of pipe

Q = flow rate, gal/min

D = inside diameter of pipe, in

C = Hazen-Williams C coefficient, dimensionless

### Equation 5 (SI Units)

Where;

Q = flow rate, m^{3}/h

D = pipe inside diameter, mm

P_{km} = frictional pressure drop, kPa/km

SG = liquid specific gravity (water = 1.00)

C = Hazen-Williams C factor, dimensionless

Common values for the Hazen-Williams coefficient C are listed in the table below:

Pipe material | C Factor |

Brick | 100 |

Cast iron (old) | 100 |

Iron (worn/pitted) | 60 – 80 |

Polyvinyl chloride (PVC) | 150 |

Smooth wood | 120 |

Smooth masonry | 120 |

Smooth pipes (all metals) | 130 – 140 |

Vitrified clay | 110 |

Read about another popular equation for head loss calculations using Darcy-Weisbach equation