Hazen–Williams equation

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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[1] such as fire sprinkler systems,[2] 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,[3] and therefore is only valid at room temperature and conventional velocities.[4]

General form[edit]

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 hydraulic head loss was proportional to the velocity squared.[5] 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.[6]

The Darcy-Weisbach equation was difficult to use because the friction factor was difficult to estimate.[7] 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.


  • V is velocity (in ft/s for US customary units, in m/s for SI units)
  • 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 (in ft for US customary units, in m for SI units)
  • 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.[8]

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.[9] The value of k is 0.001−0.04.[10]

Typical C factors used in design, which take into account some increase in roughness as pipe ages are as follows:[11]

Material C Factor low C Factor high Reference
Asbestos-cement 140 140 -
Cast iron new 130 130 [11]
Cast iron 10 years 107 113 [11]
Cast iron 20 years 89 100 [11]
Cement-Mortar Lined Ductile Iron Pipe 140 140
Concrete 100 140 [11]
Copper 130 140 [11]
Steel 90 110
Galvanized iron 120 120 [11]
Polyethylene 140 140 [11]
Polyvinyl chloride (PVC) 150 150 [11]
Fibre-reinforced plastic (FRP) 150 150 [11]

Pipe equation[edit]

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 d2 / 4, so

U.S. customary units (Imperial)[edit]

When used to calculate the pressure drop using the US customary units system, the equation is:[12]


Note: Caution with U S Customary Units is advised. The equation for head loss in pipes, also referred to as slope, S, expressed in "feet per foot of length" vs. in 'psi per foot of length' as described above, with the inside pipe diameter, d, being entered in feet vs. inches, and the flow rate, Q, being entered in cubic feet per second, cfs, vs. gallons per minute, gpm, appears very similar. However, the constant is 4.73 vs. the 4.52 constant as shown above in the formula as arranged by NFPA for sprinkler system design. The exponents and the Hazen-Williams "C" values are unchanged.

SI units[edit]

When used to calculate the head loss with the International System of Units, the equation will then become



  • S = Hydraulic slope
  • hf = head loss in meters (water) over the length of pipe
  • L = length of pipe in meters
  • Q = volumetric flow rate, m3/s (cubic meters per second)
  • C = pipe roughness coefficient
  • d = inside pipe diameter, m (meters)
Note: pressure drop can be computed from head loss as hf × the unit weight of water (e.g., 9810 N/m3 at 4 deg C)

See also[edit]


  1. ^ "Hazen–Williams Formula". Archived from the original on 22 August 2008. Retrieved 6 December 2008.
  2. ^ "Hazen–Williams equation in fire protection systems". Canute LLP. 27 January 2009. Archived from the original on 6 April 2013. Retrieved 27 January 2009.
  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. ^ Pumping station design. Jones, Garr M. (3rd ed.). Burlington, MA: Butterworth-Heinemann. 2006. p. 3.4. ISBN 978-0-08-094106-6. OCLC 144609617.{{cite book}}: CS1 maint: others (link)
  5. ^ 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, doi:10.1002/j.1551-8833.2006.tb07611.x, S2CID 108604497, p. 112.
  6. ^ Walski 2006, p. 112
  7. ^ Walski 2006, p. 113
  8. ^ Williams & Hazen 1914, 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."
  9. ^ Williams & Hazen 1914, p. 1
  10. ^ Williams & Hazen 1914, pp. 1–2
  11. ^ a b c d e f g h i j Hazen-Williams Coefficients, Engineering ToolBox, retrieved 7 October 2012
  12. ^ 2007 version of NFPA 13: Standard for the Installation of Sprinkler Systems, page 13-213, eqn
  13. ^ "Comparison of Pipe Flow Equations and Head Losses in Fittings" (PDF). Archived from the original (PDF) on 21 January 2022. Retrieved 6 December 2008.

Further reading[edit]

External links[edit]