Finding the Darcy friction factor
How the Darcy friction factor is read from a Moody chart or found with the Colebrook and Swamee-Jain equations from Reynolds number and roughness.
Why the friction factor is a separate step
The Darcy-Weisbach equation splits a friction problem into two parts. The geometry and flow, meaning length, diameter and velocity, are easy to measure. The friction factor f rolls up everything harder to see: whether the flow is laminar or turbulent, and how rough the pipe wall is relative to its bore. This calculator takes f as a given so you can pair it with any source you trust, from a textbook chart to a solver. Getting f right matters, because head loss is directly proportional to it.
Laminar flow has an exact factor
When the Reynolds number is below about 2300 the flow is laminar and orderly, and the friction factor no longer depends on roughness at all. It is simply 64 divided by the Reynolds number. This is one of the few clean results in pipe flow, so if you know the velocity, diameter and kinematic viscosity you can compute f by hand and drop it straight into the head loss box. Above roughly 4000 the flow is turbulent and the picture changes.
Turbulent flow and the Colebrook equation
In turbulent flow the friction factor depends on both the Reynolds number and the relative roughness, the wall roughness divided by the diameter. The Colebrook equation ties these together, but it is implicit, meaning f appears on both sides and you must iterate to solve it. That is exactly what a Moody chart plots graphically: pick your Reynolds number along the bottom, follow the curve for your relative roughness, and read f off the left axis. The Swamee-Jain equation is a popular explicit fit that returns f in one pass with no iteration.
From factor to head loss
Once you have f, the rest is arithmetic that this tool does for you. It multiplies f by the length over diameter ratio and by velocity squared over two g to get the head loss in metres. Adding a density converts that height into a pressure drop. Because f is dimensionless, the only thing you must keep consistent is the SI units on the other inputs, so convert a bore given in millimetres to metres before you start.