Hazen-Williams C factors by material and condition

These are the standard Williams and Hazen tabulation as reproduced in Crane TP-410 and in normal water-industry practice, including AWWA material and the Mays Water Distribution Systems Handbook. Where a Fluid Network Studio pipe is set to the Hazen-Williams method and has no C of its own, it uses 130, which is the value for new cast or ductile iron.

Selecting a preset fills C and leaves it editable. That is deliberate. The presets are a sensible starting set, and published tabulations differ by a few points for the same material, which is itself a fair signal of how precise this coefficient really is.

The Hazen-Williams calculator carries a wider table of typical ranges by material, and the assigned C values that NFPA 13 mandates for fire sprinkler hydraulic calculations. If you are working on a sprinkler submission, use the assigned values there rather than the engineering presets here.

The equation these coefficients belong to

A C value is only meaningful with the equation and the unit system it was fitted for. In SI, with head loss in m, length in m, flow in m^3/s and internal diameter in m:

h_f = 10.674 * L * Q^1.852 / ( C^1.852 * D^4.871 )

Note what is missing. There is no friction factor, no Reynolds number, no viscosity and no gravity term, because the constant 10.674 already carries standard gravity and the correlation was fitted at ordinary water temperatures. That absence is exactly why the method is convenient and exactly why it is limited.

On the constant. Many textbooks round this to 10.67 with an exponent of 4.87. The form used above, 10.674 with 4.871, is the one given in the EPANET 2 Users Manual, and it is what the Fluid Network Studio solver, the calculator and the published verification cases all use, so every page on this site quotes the same number. The two forms differ by about 0.2 per cent, which is negligible next to the uncertainty in C itself, but a reference page should say which one it means.

The same equation in US customary units carries a different leading constant, so a C value is portable between unit systems while the constant is not. If you are checking someone else's spreadsheet and the answer is out by a large factor, the leading constant is the first place to look.

C is not a roughness, and there is no clean conversion

People often ask for a table converting C to absolute roughness. There is no honest one, and it is worth being precise about why.

Darcy-Weisbach separates the problem into a geometry term and a friction factor, and the friction factor depends on both relative roughness and Reynolds number. Hazen-Williams has no Reynolds number at all. Its exponents, 1.852 on flow and 4.871 on diameter, are fitted values chosen so that one coefficient reproduces measured head loss for water over the range of pipes and velocities the original data covered. C therefore absorbs roughness, viscosity and the flow regime together.

The consequence is that any C-to-roughness mapping is only valid at one diameter and one velocity. Move to a different pipe size or a different duty and the equivalence shifts. If you need a conversion for a particular line, the defensible method is to match the two head losses at your actual diameter and flow rather than to look up a pair in a table. That is a calculation, not a constant.

If you want a roughness, use the absolute roughness reference and Darcy-Weisbach directly.

Where Hazen-Williams is valid, and where it is not

Hazen-Williams is calibrated for water at ordinary temperatures, roughly 4 to 25 degrees C, flowing turbulently in pressurised pipe. Inside that fence it is a long-standing and perfectly respectable method, embedded in water distribution practice and in fire-protection codes.

Outside it, the method has no physical basis to fall back on.

• Other fluids. It is not valid for oils, glycols, brines, slurries or any liquid whose viscosity differs materially from water. There is no viscosity term to correct.
• Gases. It is not valid at all. Use the compressible methods.
• Hot or very cold water. The correlation cannot see temperature, so it cannot correct for the viscosity change. Above roughly 25 degrees C or near freezing, prefer Darcy-Weisbach.
• Laminar and transitional flow. The fitted exponents describe turbulent water. In small-bore low-velocity lines the method drifts away from reality.
• Very high or very low velocity. The original data covered ordinary distribution velocities. Extrapolating far outside them is extrapolating a curve fit.

Darcy-Weisbach with Colebrook-White has none of these limits, which is why it is the default method in Fluid Network Studio and why Hazen-Williams is offered as an explicit opt-in for the water networks where it is the local convention.

How C changes with age

This is the part of the subject that a table cannot settle for you, and the part that matters most.

The tabulated C values for new pipe are reasonably firm. What happens afterwards depends on water chemistry, velocity, temperature and whether the pipe is lined, and the spread is enormous. Unlined iron in aggressive water can lose forty or fifty points of C over a few decades through tuberculation, which is why the table above carries separate entries for cast iron at roughly ten and twenty years and a further one for old tuberculated iron. Lined and plastic pipes barely move, which is a large part of why they are specified.

Two practical rules follow.

First, design for the pipe you will have at the end of the design life, not the pipe you will commission. Sizing a distribution main on a new-pipe C and then watching it fail its duty in year twenty is a well-documented way to be wrong.

Second, if the system exists and you can measure it, do not use this table at all. A flow and a differential pressure on a known length of main gives you the C of that main, which beats any published value for its material. Rearranging the equation above for C is straightforward, and a calibrated coefficient is the only genuinely defensible one.

Frequently asked questions

What is a typical Hazen-Williams C value?

Around 150 for new plastic and for lined pipe, 140 for cement-lined ductile iron, 130 for new cast or ductile iron, 120 for new steel and for concrete, and down towards 100 or below for old unlined metal mains. The full preset list is in the table above, and the Hazen-Williams calculator carries typical ranges as well as single values.

Is a higher C smoother or rougher?

Smoother. C sits in the denominator of the head-loss equation, so a higher C gives less head loss. This is the opposite convention to absolute roughness, where a higher number means a rougher pipe, and mixing the two up is a common slip.

Should I use Hazen-Williams or Darcy-Weisbach?

Darcy-Weisbach, unless you have a specific reason not to. It is physically based, valid for any Newtonian fluid, temperature and flow regime, and it separates roughness from the flow regime rather than merging them. Hazen-Williams earns its place where the local convention, a client standard or a code requires it, or where you are checking against an existing model built with it. Fluid Network Studio solves networks with either method.

Does the C value depend on pipe diameter?

In principle yes, mildly, and in practice the tabulated values are treated as diameter-independent. This is one of the approximations built into the method. It is also one of the reasons a C fitted from measurements on one main should not be carried across to a main of a very different size without thought.

Can I use Hazen-Williams for compressed air or gas?

No. The correlation is a water fit with no density or compressibility term, and it will give a confidently wrong answer for a gas. Compressed air and process gas need the compressible treatment, which Fluid Network Studio solves in pressure-squared form with the Churchill friction factor.

Which C does the solver use if I do not set one?

130, the value for new cast or ductile iron, applied to any pipe on the Hazen-Williams method that has no coefficient of its own. It is a deliberate mid-range default rather than an optimistic one, but it is still a default, so set the coefficient explicitly on anything that matters.

References

• Williams, G. S. and Hazen, A., Hydraulic Tables, John Wiley and Sons. The original tabulation the coefficient comes from.
• Rossman, L. A. (2000), EPANET 2 Users Manual, US Environmental Protection Agency. The source of the SI constant 10.674 and the exponent 4.871 used here. epa.gov/water-research/epanet
• Crane Co., Technical Paper No. 410: Flow of Fluids Through Valves, Fittings and Pipe. tp410.com
• Mays, L. W. (editor), Water Distribution Systems Handbook, and AWWA practice material such as AWWA M11, for the material and condition values used in water-industry practice.

Related

• Hazen-Williams equation calculator, with the wider typical-range table and the NFPA 13 assigned values.
• Absolute pipe roughness values by material, the Darcy-Weisbach equivalent input.
• Pipe schedule dimensions, for the internal diameter the fourth-power term needs.
• Hazen-Williams distribution worked example and the verification page, where the Hazen-Williams implementation is cross-checked against OWA-EPANET 2.2.
• Open the Studio to solve a whole network with either head-loss method.