# Friction Factor Calculator

URL: https://speedycalculator.com/calculators/physics/friction-factor/
Last updated: 2026-10-30

> The friction factor calculator finds the Darcy friction factor for pipe flow from the Reynolds number and relative roughness – laminar (64/Re) or turbulent with the Swamee–Jain and Haaland approximations of the Colebrook equation.

## Inputs

- Reynolds number from
- Reynolds number
- Velocity (m/s, km/h, mph, ft/s, knots)
- Kinematic viscosity (mm²/s (cSt), m²/s)
- Inner diameter D (mm, cm, m, in, µm)
- Pipe material (roughness ε)
- Custom roughness ε (mm, cm, m, in, µm)

## Results

- Re
- Relative roughness ε/D
- Darcy friction factor
- Haaland friction factor
- Fanning friction factor
- Flow regime

## 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]
```

## Explanation

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

FieldWhat to enterReynolds number fromChoose one: Direct value, Velocity and kinematic viscosityReynolds numberA numberVelocityUnits: m/s, km/h, mph, ft/s, knotsKinematic viscosityUnits: mm²/s (cSt), m²/sInner diameter DUnits: mm, cm, m, in, µmPipe material (roughness ε)Choose one: Drawn tubing, glass, plastic, Commercial steel, Galvanised iron, Cast iron …Custom roughness εUnits: 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 numberDarcy friction factor50,0000.020975,0000.0191100,0000.018125,0000.0173150,0000.0167200,0000.0158

## 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.

## FAQ

**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.


## Sources

- [Fluid Mechanics (F. M. White) – reference text](https://www.mheducation.com/highered/product/fluid-mechanics-white-chul/M9781260258318.html)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)