The friction factor is the key unknown in pipe-flow calculations. The Colebrook equation gives it implicitly; Swamee–Jain and Haaland provide explicit approximations within about 1–2%.
How to use the Friction Factor Calculator
- Reynolds number from
- Reynolds number
- Velocity
- Kinematic viscosity
- Inner diameter D
- Pipe material (roughness ε)
- Custom roughness ε
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the Reynolds number (or compute it from velocity and viscosity), the pipe diameter and roughness.
Inputs explained
| Field | What to enter |
|---|---|
| Reynolds number from | Choose one: Direct value, Velocity and kinematic viscosity |
| Reynolds number | A number |
| Velocity | Units: m/s, km/h, mph, ft/s, knots |
| Kinematic viscosity | Units: mm²/s (cSt), m²/s |
| Inner diameter D | Units: mm, cm, m, in, µm |
| Pipe material (roughness ε) | Choose one: Drawn tubing, glass, plastic, Commercial steel, Galvanised iron, Cast iron … |
| Custom roughness ε | Units: mm, cm, m, in, µm |
What each result means
- Relative roughness ε/D = [eps]÷[Inner diameter D]
- Fanning friction factor = [Darcy friction factor]÷4
Example values
How the main result changes when Reynolds number changes and everything else stays at the example values above:
| Reynolds number | Darcy friction factor |
|---|---|
| 50,000 | 0.0209 |
| 75,000 | 0.0191 |
| 100,000 | 0.018 |
| 125,000 | 0.0173 |
| 150,000 | 0.0167 |
| 200,000 | 0.0158 |
Formula
Laminar: f = 64/Re · Swamee–Jain: f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re^{0.9})]² · Haaland: 1/√f = −1.8·log₁₀[(ε/D/3.7)^{1.11} + 6.9/Re]
Worked example
Inputs — Reynolds number from: Direct value; Reynolds number: 100,000; Inner diameter D: 50 mm; Pipe material (roughness ε): Drawn tubing, glass, plastic.
Results — Re = 100,000; Relative roughness ε/D = 0.00003; Darcy friction factor = 0.01805; Haaland friction factor = 0.01794; Fanning friction factor = 0.00451; Flow regime = Turbulent.
Key terms
- Relative roughness
- Wall roughness divided by diameter.
Frequently asked questions
What is the Colebrook equation?
An implicit formula for turbulent friction factor in rough pipes.
How accurate is Swamee–Jain?
Within about 1% of Colebrook for 5000 < Re < 10⁸.
Why does roughness matter only in turbulent flow?
Laminar flow is insensitive to wall roughness.
What happens between Re 2300 and 4000?
Flow is unstable and f is uncertain.