The Hazen–Williams equation is an empirical relationship which relates the flow of water in a pipe with the physical properties of the pipe and the pressure drop caused by friction. It is used in the design of water pipe systems such as fire sprinkler systems, water supply networks, and irrigation systems. It is named after Allen Hazen and Gardner Stewart Williams.

The Hazen–Williams equation has the advantage that the coefficient C is not a function of the Reynolds number, but it has the disadvantage that it is only valid for water. Also, it does not account for the temperature or viscosity of the water.

## Hazen–Williams equation: General form

Henri Pitot discovered that the velocity of a fluid was proportional to the square root of its head in the early 18th century. It takes energy to push a fluid through a pipe, and Antoine de Chézy discovered that the head loss was proportional to the velocity squared. Consequently, the Chézy formula relates hydraulic slope S (head loss per unit length) to the fluid velocity V and hydraulic radius R:

The variable C expresses the proportionality, but the value of C is not a constant. In 1838 and 1839, Gotthilf Hagen and Jean Léonard Marie Poiseuille independently determined a head loss equation for laminar flow, the Hagen–Poiseuille equation. Around 1845, Julius Weisbach and Henry Darcy developed the Darcy–Weisbach equation.

The Darcy-Weisbach equation was difficult to use because the friction factor was difficult to estimate. In 1906, Hazen and Williams provided an empirical formula that was easy to use. The general form of the equation relates the mean velocity of water in a pipe with the geometric properties of the pipe and slope of the energy line.

where:

• V is velocity
• k is a conversion factor for the unit system (k = 1.318 for US customary units, k = 0.849 for SI units)
• C is a roughness coefficient
• R is the hydraulic radius
• S is the slope of the energy line (head loss per length of pipe or hf/L)

The equation is similar to the Chézy formula but the exponents have been adjusted to better fit data from typical engineering situations. A result of adjusting the exponents is that the value of C appears more like a constant over a wide range of the other parameters.

The conversion factor k was chosen so that the values for C were the same as in the Chézy formula for the typical hydraulic slope of S=0.001. The value of k is 0.001.

Typical C factors used in design, which take into account some increase in roughness as pipe ages are as follows:

Material C Factor low C Factor high Reference
Asbestos-cement 140 140 -
Cast iron new 130 130
Cast iron 10 years 107 113
Cast iron 20 years 89 100
Cast iron 30 years 75 90
Cast iron 40 years 64 83
Cement-Mortar Lined Ductile Iron Pipe 140 140 -
Concrete 100 140
Copper 130 140
Steel 90 110 -
Galvanized iron 120 120
Polyethylene 140 140
Polyvinyl chloride (PVC) 150 150
Fibre-reinforced plastic (FRP) 150 150

## Hazen–Williams equation: Pipe equation

The general form can be specialized for full pipe flows. Taking the general form

and exponentiating each side by 1/0.54 gives (rounding exponents to 3-4 decimals)

Rearranging gives

The flow rate Q = V A, so

The hydraulic radius R (which is different from the geometric radius r) for a full pipe of geometric diameter d is d/4; the pipe's cross sectional area A is π d / 4, so

### Hazen–Williams equation: U.S. customary units (Imperial)

When used to calculate the pressure drop using the US customary units system, the equation is:

where:

• Spsi per foot = frictional resistance (pressure drop per foot of pipe) in psig/ft (pounds per square inch gauge pressure per foot)
• Pd = pressure drop over the length of pipe in psig (pounds per square inch gauge pressure)
• L = length of pipe in feet
• Q = flow, gpm (gallons per minute)
• C = pipe roughness coefficient
• d = inside pipe diameter, in (inches)

### Hazen–Williams equation: SI units

When used to calculate the head loss with the International System of Units, the equation becomes:

where:

• S = Hydraulic slope
• hf = head loss in meters (water) over the length of pipe
• L = length of pipe in meters
• Q = volumetric flow rate, m/s (cubic meters per second)
• C = pipe roughness coefficient
• d = inside pipe diameter, m (meters)

• Fluid dynamics
• Friction
• Pressure
• Prony equation
• Volumetric flow rate
• Water pipe

## Hazen–Williams equation: References

1. "Hazen–Williams Formula". Retrieved 2008-12-06.
2. "Hazen–Williams equation in fire protection systems". Canute LLP. 27 January 2009. Archived from the original on 2013-04-06. Retrieved 2009-01-27.
3. Brater, Ernest F.; King, Horace W.; Lindell, James E.; Wei, C. Y. (1996). "6". Handbook of Hydraulics (Seventh ed.). New York: McGraw Hill. p. 6.29. ISBN 0-07-007247-7.
4. Walski, Thomas M. (March 2006), "A history of water distribution", Journal of the American Water Works Association, American Water Works Association, 98 (3): 110–121, p. 112.
5. , p. 112
6. , p. 113
7. , p. 1, stating "Exponents can be selected, however, representing approximate average conditions, so that the value of c for a given condition of surface will vary so little as to be practically constant."
8. , p. 1
9. , pp. 1–2
10. Hazen-Williams Coefficients, Engineering ToolBox, retrieved 7 October 2012
11. 2007 version of NFPA 13: Standard for the Installation of Sprinkler Systems, page 13-213, eqn 22.4.2.1
12. "Comparison of Pipe Flow Equations and Head Losses in Fittings" (PDF). Retrieved 2008-12-06.
• Finnemore, E. John; Franzini, Joseph B. (2002), Fluid Mechanics (10th ed.), McGraw Hill
• Mays, Larry W. (1999), Hydraulic Design Handbook, McGraw Hill
• Watkins, James A. (1987), Turf Irrigation Manual (5th ed.), Telsco
• Williams, Gardner Stewart; Hazen, Allen (1905), Hydraulic tables: showing the loss of head due to the friction of water flowing in pipes, aqueducts, sewers, etc. and the discharge over weirs (first ed.), New York: John Wiley and Sons
• Williams and Hazen, Second edition, 1909
• Williams, Gardner Stewart; Hazen, Allen (1914), Hydraulic tables: the elements of gagings and the friction of water flowing in pipes, aqueducts, sewers, etc., as determined by the Hazen and Williams formula and the flow of water over sharp-edged and irregular weirs, and the quantity discharged as determined by Bazin's formula and experimental investigations upon large models. (2nd revised and enlarged ed.), New York: John Wiley and Sons
• Williams, Gardner Stewart; Hazen, Allen (1920), Hydraulic tables: the elements of gagings and the friction of water flowing in pipes, aqueducts, sewers, etc., as determined by the Hazen and Williams formula and the flow of water over sharp-edged and irregular weirs, and the quantity discharged as determined by Bazin's formula and experimental investigations upon large models. (3rd ed.), New York: John Wiley and Sons, OCLC 1981183
• Engineering Toolbox reference
• Engineering toolbox Hazen–Williams coefficients
• Online Hazen–Williams calculator for gravity-fed pipes.
• Online Hazen–Williams calculator for pressurized pipes.