How it’s calculated
The Darcy–Weisbach equation gives the frictional loss in a straight, full, circular pipe with steady, incompressible, fully developed flow. The friction factor comes from the Reynolds number and the relative roughness: 64/Re for laminar flow, and the explicit Swamee–Jain (1976) approximation of the Colebrook equation otherwise (within about 1% of Colebrook for 5,000 ≤ Re ≤ 10⁸ and 10⁻⁶ ≤ ε/D ≤ 10⁻²).
Fittings, valves, entrances and elevation changes are not included; add them as minor losses (K·v²/2g) and static head separately.
Example: 2 L/s of 20 °C water in 100 m of 50 mm commercial steel pipe. v = 0.002 ÷ (π × 0.05²/4) = 1.019 m/s, Re = 1000 × 1.019 × 0.05 ÷ 0.001002 ≈ 50,800, ε/D = 0.046/50 = 0.00092, f ≈ 0.0239, h_f = 0.0239 × (100/0.05) × 1.019² ÷ (2 × 9.807) ≈ 2.53 m, so Δp ≈ 24.8 kPa (3.6 psi).
| Material (new) | ε (mm) |
|---|---|
| Drawn tubing | 0.0015 |
| Commercial steel / wrought iron | 0.046 |
| Asphalted cast iron | 0.12 |
| Galvanized iron | 0.15 |
| Cast iron | 0.26 |
| Wood stave | 0.18 – 0.9 |
| Concrete | 0.3 – 3.0 |
| Riveted steel | 0.9 – 9.0 |
Frequently asked questions
What is the difference between head loss and pressure drop?
They are the same loss in different units: Δp = ρ·g·h_f. For water, 1 m of head ≈ 9.81 kPa and 1 ft ≈ 0.433 psi.
Why Swamee–Jain instead of Colebrook?
Colebrook is implicit and needs iteration. Swamee–Jain is explicit and agrees with it to about 1% across normal engineering ranges, well inside the uncertainty of pipe roughness itself.
Is this the Darcy or Fanning friction factor?
Darcy (Moody) friction factor. The Fanning factor is one quarter of it.
How do I include fittings?
Add K·v²/(2g) for each fitting, or convert fittings to an equivalent length and add it to L.
Embed this calculator
Add this free calculator to your own website. Copy the code below into your page’s HTML:
Sources
- DOE Fundamentals Handbook: Thermodynamics, Heat Transfer, and Fluid Flow, Vol. 3 (Head Loss, Darcy’s equation) — U.S. Department of Energy
- Pipe Flows: Losses, Table 11.1 pipe wall roughness (ME 308 course notes, C. Wassgren) — Purdue University
- Slurry Transport (Miedema), 3.2 The Darcy–Weisbach Friction Factor (Swamee–Jain) — Engineering LibreTexts
- University Physics Vol. 1, 14.7 Viscosity and Turbulence (Table 14.4) — OpenStax
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
Spotted a mistake or missing option? Report a problem · GitHub issue· Suggest a calculator