Darcy friction factor formulae
In fluid dynamics, the Darcy friction factor formulae are equations — based on experimental data and theory — for the Darcy friction factor. The Darcy friction factor is a dimensionless quantity used in the Darcy–Weisbach equation, for the description of friction losses in pipe flow as well as open channel flow. It is also known as the Darcy–Weisbach friction factor or Moody friction factor and is four times larger than the Fanning friction factor.^{[1]}
Contents 
Flow regime
Which friction factor formula may be applicable depends upon the type of flow that exists:
 Laminar flow
 Transition between laminar and turbulent flow
 Fully turbulent flow in smooth conduits
 Fully turbulent flow in rough conduits
 Free surface flow.
Laminar flow
The Darcy friction factor for laminar flow (Reynolds number less than 2000) is given by the following formula:
where:
 f is the Darcy friction factor
 Re is the Reynolds number.
Transition flow
Transition (neither fullylaminar nor fullyturbulent) flow occurs in the range of Reynolds numbers between 2300 and 4000. The value of the Darcy friction factor may be subject to large uncertainties in this flow regime.
Turbulent flow in smooth conduits
Empirical correlations exist for this flow regime. Such correlations are included in the ASHRAE Handbook of Fundamentals.
Turbulent flow in rough conduits
The Darcy friction factor for fully turbulent flow (Reynolds number greater than 4000) in rough conduits is given by the Colebrook equation.
Free surface flow
The last formula in the Colebrook equation section of this article is for free surface flow. The approximations elsewhere in this article are not applicable for this type of flow.
Choosing a formula
Before choosing a formula it is worth knowing that in the paper on the Moody chart, Moody stated the accuracy is about ±5% for smooth pipes and ±10% for rough pipes. If more than one formula is applicable in the flow regime under consideration, the choice of formula may be influenced by one or more of the following:
 Required precision
 Speed of computation required
 Available computational technology:

 calculator (minimize keystrokes)
 spreadsheet (singlecell formula)
 programming/scripting language (subroutine).
Colebrook equation
Compact forms
The Colebrook equation is an implicit equation that combines experimental results of studies of turbulent flow in smooth and rough pipes. It was developed in 1939 by C. F. Colebrook.^{[2]} The 1937 paper by C. F. Colebrook and C. M. White^{[3]} is often erroneously cited as the source of the equation. This is partly because Colebrook in a footnote (from his 1939 paper) acknowledges his debt to White for suggesting the mathematical method by which the smooth and rough pipe correlations could be combined. The equation is used to iteratively solve for the Darcy–Weisbach friction factor f. This equation is also known as the Colebrook–White equation.
For conduits that are flowing completely full of fluid at Reynolds numbers greater than 4000, it is defined as:
 or
where:
 f is the Darcy friction factor
 Roughness height, (m, ft)
 Hydraulic diameter, D_{h} (m, ft) — For fluidfilled, circular conduits, D_{h} = D = inside diameter
 Hydraulic radius, R_{h} (m, ft) — For fluidfilled, circular conduits, R_{h} = D/4 = (inside diameter)/4
 Re is the Reynolds number.
Solving
Due to the implicit nature of the Colebrook equation, determination of a friction factor requires some iteration or a numerical solving method.
Expanded forms
Additional, mathematicallyequivalent forms of the Colebrook equation are:

 where:
 1.7384... = 2 log (2 * 3.7) = 2 log (7.4)
 18.574 = 2.51 * 3.7 * 2
 where:
and
 or

 where:
 1.1364... = 1.7384...  2 log (2) = 2 log (7.4)  2 log (2) = 2 log (3.7)
 9.287 = 18.574 / 2 = 2.51 * 3.7.
 where:
The additional equivalent forms above assume that the constants 3.7 and 2.51 in the formula at the top of this section are exact. The constants are probably values which were rounded by Colebrook during his curve fitting; but they are effectively treated as exact when comparing (to several decimal places) results from explicit formulae (such as those found elsewhere in this article) to the friction factor computed via Colebrook's implicit equation.
Equations similar to the additional forms above (with the constants rounded to fewer decimal places—or perhaps shifted slightly to minimize overall rounding errors) may be found in various references. It may be helpful to note that they are essentially the same equation.
Free surface flow
Another form of the ColebrookWhite equation exists for free surfaces. Such a condition may exist in a pipe that is flowing partiallyfull of fluid. For free surface flow:
Approximations of the Colebrook equation
Haaland equation
The Haaland equation is used to solve directly for the Darcy–Weisbach friction factor f for a fullflowing circular pipe. It is an approximation of the implicit Colebrook–White equation, but the discrepancy from experimental data is well within the accuracy of the data. It was developed by S. E. Haaland in 1983.
The Haaland equation is defined as:
where:
 f is the Darcy friction factor
 is the relative roughness
 Re is the Reynolds number.
Swamee–Jain equation
The Swamee–Jain equation is used to solve directly for the Darcy–Weisbach friction factor f for a fullflowing circular pipe. It is an approximation of the implicit Colebrook–White equation.
where f is a function of:
 Roughness height, ε (m, ft)
 Pipe diameter, D (m, ft)
 Reynolds number, Re (unitless).
Serghides's solution
Serghides's solution is used to solve directly for the Darcy–Weisbach friction factor f for a fullflowing circular pipe. It is an approximation of the implicit Colebrook–White equation. It was derived using Steffensen's method.^{[4]}
The solution involves calculating three intermediate values and then substituting those values into a final equation.
where f is a function of:
 Roughness height, ε (m, ft)
 Pipe diameter, D (m, ft)
 Reynolds number, Re (unitless).
The equation was found to match the Colebrook–White equation within 0.0023% for a test set with a 70point matrix consisting of ten relative roughness values (in the range 0.00004 to 0.05) by seven Reynolds numbers (2500 to 10^{8}).
GoudarSonnad equation
Goudar equation is the most accurate approximation to solve directly for the Darcy–Weisbach friction factor f for a fullflowing circular pipe. It is an approximation of the implicit Colebrook–White equation. Equation has the following form ^{[5]}
 s = bd + ln(d)
 q = s^{(s / (s + 1))}
where f is a function of:
 Roughness height, ε (m, ft)
 Pipe diameter, D (m, ft)
 Reynolds number, Re (unitless).
Blasius correlations
Early approximations by Blasius are given in terms of the Fanning friction factor in the Paul Richard Heinrich Blasius article.
References
 ^ Manning, Francis S.; Thompson, Richard E. (1991). Oilfield Processing of Petroleum. Vol. 1: Natural Gas. PennWell Books. ISBN 0878143432, 420 pages. See page 293.
 ^ Colebrook, C.F. (February 1939). "Turbulent flow in pipes, with particular reference to the transition region between smooth and rough pipe laws". Journal of the Institution of Civil Engineers (London).
 ^ Colebrook, C. F. and White, C. M. (1937). "Experiments with Fluid Friction in Roughened Pipes". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 161 (906): 367–381.
 ^ Serghides, T.K (1984). "Estimate friction factor accurately". Chemical Engineering Journal 91(5): 63–64.
 ^ Goudar, C.T., Sonnad, J.R. (August 2008). "Comparison of the iterative approximations of the ColebrookWhite equation". Hydrocarbon Processing Fluid Flow and Rotating Equipment Special Report(August 2008): 79–83.
Further reading
 Colebrook, C.F. (February 1939). "Turbulent flow in pipes, with particular reference to the transition region between smooth and rough pipe laws". Journal of the Institution of Civil Engineers (London).
For the section which includes the freesurface form of the equation — Computer Applications in Hydraulic Engineering (5th ed.). Haestad Press. 2002, p. 16.  Haaland, SE (1983). "Simple and Explicit Formulas for the Friction Factor in Turbulent Flow". Journal of Fluids Engineering (ASME) 103 (5): 89–90.
 Swamee, P.K.; Jain, A.K. (1976). "Explicit equations for pipeflow problems". Journal of the Hydraulics Division (ASCE) 102 (5): 657–664.
 Serghides, T.K (1984). "Estimate friction factor accurately". Chemical Engineering 91 (5): 63–64. — Serghides' solution is also mentioned here.
 Moody, L.F. (1944). "Friction Factors for Pipe Flow". Transactions of the ASME 66 (8): 671–684.