Friction in pipes turns pressure into heat. The Darcy–Weisbach equation is the most accurate general way to estimate it for any fluid, as long as you know the friction factor (from the Moody chart or the friction factor calculator).
How to use the Darcy–Weisbach Calculator
- Pipe length L
- Inner diameter D
- Flow velocity v
- Fluid density ρ
- Darcy friction factor f
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the pipe length and inner diameter, the flow velocity, fluid density and the Darcy friction factor.
Inputs explained
| Field | What to enter |
|---|---|
| Pipe length L | Units: m, cm, mm, km, in, ft … |
| Inner diameter D | Units: mm, cm, m, in, µm |
| Flow velocity v | Units: m/s, km/h, mph, ft/s, knots |
| Fluid density ρ | Units: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³ |
| Darcy friction factor f | A number |
What each result means
- Pressure drop ΔP = [Darcy friction factor f]×[Pipe length L]÷[Inner diameter D]×[Fluid density ρ]×[Flow velocity v]^2÷2
- Head loss = [Darcy friction factor f]×[Pipe length L]÷[Inner diameter D]×[Flow velocity v]^2÷(2×9.80665)
- Flow rate = [Flow velocity v]×pi×[Inner diameter D]^2÷4
Example values
How the main result changes when Pipe length L changes and everything else stays at the example values above:
| Pipe length L (m) | Pressure drop ΔP (kPa) |
|---|---|
| 50 | 22.4595 |
| 75 | 33.6893 |
| 100 | 44.919 |
| 125 | 56.1488 |
| 150 | 67.3785 |
| 200 | 89.838 |
Formula
ΔP = f·(L/D)·ρv²/2, h_f = ΔP/(ρg)
Worked example
Inputs — Pipe length L: 100 m; Inner diameter D: 50 mm; Flow velocity v: 1.5 m/s; Fluid density ρ: 998.2 kg/m³; Darcy friction factor f: 0.02.
Results — Pressure drop ΔP = 44.919 kPa; Head loss = 4.58872 m; Flow rate = 2.94524 L/s.
Key terms
- Head loss
- Energy lost per unit weight, in metres.
Frequently asked questions
Is the Darcy friction factor the same as Fanning?
No – Darcy f = 4 × Fanning f.
What is a typical f?
0.015–0.03 for turbulent water flow in commercial pipes.
How does velocity affect the loss?
Roughly with its square.
Does it include fittings?
No – add minor losses (K·v²/2g) separately.