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Updated August 2026<\/p>\n

A friction loss formula is a calculation that converts resistance inside a pipe into head loss or pressure drop. For ordinary water service, the two formulas you will see most often are Darcy-Weisbach and Hazen-Williams. Darcy-Weisbach starts from velocity, pipe geometry, viscosity, and roughness. Hazen-Williams replaces those fluid and surface terms with an empirical C-factor. Both can produce a useful design estimate, but only when the inputs, unit form, and operating range are stated.<\/p>\n

This guide works through both methods line by line. It also shows what neither straight-pipe equation includes: valves, fittings, elevation, required discharge pressure, and the pump’s full operating curve. If you already know your inputs and only need a quick numerical result, use BBP’s friction loss calculator<\/a>. This article owns the method-choice and audit trail; the calculator owns interactive calculation.<\/p>\n

Scope:<\/strong> the examples below cover ordinary water service at a declared duty point. They do not establish a shortcut for slurry, pulp stock, non-Newtonian liquids, or an old pipe whose internal condition is unknown. A calculation can be perfectly repeatable and still miss field resistance if the flow, inside diameter, viscosity, roughness, C-factor, or fitting data do not describe the installed line.<\/p>\n

This is also not the fire hose coefficient formula that dominates the current search results. A hose friction shortcut is built around different coefficients, units, and operating practice; do not insert its C value into either pipe method below. If you are comparing a friction loss formula in pipe design, a friction loss coefficient, or a friction loss formula Hazen-Williams reference, keep the method’s own coefficient definition and units attached to the calculation.<\/p>\n

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Re < 2,000<\/strong>Laminar tendency<\/span><\/div>\n
Re 2,000-4,000<\/strong>Transition zone<\/span><\/div>\n
Re > 4,000<\/strong>Turbulent tendency<\/span><\/div>\n
D = inside diameter<\/strong>Not nominal size<\/span><\/div>\n<\/div>\n

Which Friction Loss Formula Should You Use?<\/h2>\n
\"Which<\/figure>\n

Use Darcy-Weisbach when the fluid properties, temperature, or pipe condition matter, when the liquid is not plain water, or when you need a method that exposes the flow regime. Use Hazen-Williams for turbulent water service when a defensible C-factor and the correct unit-specific equation are already part of the design basis. Do not choose a formula simply because one calculator asks for fewer inputs.<\/p>\n

The U.S. Environmental Protection Agency’s EPANET page<\/a> lists both Darcy-Weisbach and Hazen-Williams as available friction head-loss methods for water-distribution modeling. That is evidence that both methods have legitimate uses. It is not evidence that the methods share the same assumptions or can be swapped after a calculation has started.<\/p>\n

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Choose Darcy-Weisbach when…<\/h3>\n