Friction Factor Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Fluid calculations are especially sensitive to density, viscosity, geometry, pressure conventions, and unit systems. Friction Factor Calculator narrows that problem to the relationship used on this page, making the displayed darcy friction factor easier to audit against the inputs and the governing equation.

What this calculator does

The Friction Factor Calculator uses Hydraulic diameter (D), Surface roughness (k), Reynolds number (Re) to estimate the page’s Darcy friction factor from the fluid-mechanics relationship below. It is meant for a defined geometry and property set, so the useful part is not just the headline number but also whether your density, viscosity, dimensions, pressure reference, and flow convention match the model.

How to use it

Start with the fields that actually drive this result: Hydraulic diameter (D), Surface roughness (k), Reynolds number (Re). Keep units consistent with the menus beside the fields and avoid mixing values measured under different conditions. After calculating, change one input at a time if you are comparing scenarios; that makes cause-and-effect much easier to see.

How the calculation works

For laminar flow the page uses f = 64/Re. In turbulent flow it uses an explicit rough-pipe approximation similar to Swamee–Jain: f = 0.25/[log10(ε/(3.7D) + 5.74/Re^0.9)]².

Example

Using the page’s default example (Hydraulic diameter (D) = 0.1 m; Surface roughness (k) = 4.5e-05 m; Reynolds number (Re) = 100000.0), the calculator reports Darcy friction factor of 0.020196. Change one driving input at a time and confirm the result moves in the direction predicted by the equation; that is a quick way to catch a unit or mode mistake.

How to interpret the result

Interpret the Darcy friction factor within the fluid, geometry, pressure reference, and property values you entered. A numerically plausible answer can still be physically wrong if gauge/absolute pressure, diameter/radius, viscosity type, or unit convention is mismatched, so compare the result with the assumptions as well as the formula.

Limitations and notes

The turbulent expression is an explicit approximation and assumes an appropriate relative roughness. Transition flow near the laminar/turbulent boundary is not captured by a single sharp formula, so borderline cases deserve a Moody-chart or standards check.