Pipe Pressure Drop Calculator
Calculate the frictional pressure drop in a straight pipe, plus fittings, using the Darcy Weisbach equation.
- Re = ρvD/μ = 172,300 (turbulent, Swamee Jain)
- f = 0.01889
- ΔP = f × (L/D) × ρv²/2 + ΣK × ρv²/2 = 26.37 kPa
Darcy Weisbach Equation
Turbulent: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re0.9)]²
Laminar: f = 64 ÷ Re
f is the Darcy friction factor (Swamee Jain approximation of Colebrook, within about 1 %), ε the absolute roughness and ΣK the sum of fitting loss coefficients (for example about 0.3 to 0.9 per elbow).
Worked example
50 m³/h of water in 100 m of 4 inch Sch 40 commercial steel: v = 1.69 m/s, Re ≈ 172,000, f ≈ 0.0189, ΔP ≈ 26.4 kPa (0.26 bar).
Frequently Asked Questions
What is the Darcy friction factor?
A dimensionless factor that depends on Reynolds number and relative roughness; it is four times the Fanning friction factor.
How accurate is the Swamee Jain equation?
It matches the Colebrook equation within about 1 % for 5000 < Re < 10⁸ and typical roughness.
How do I include valves and bends?
Add their K values in the fittings box. For example a fully open gate valve is about 0.15 and a 90° long radius elbow about 0.3.
Learn more on the blog
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