Pipe Pressure Drop Calculator
Pipe pressure drop follows Darcy–Weisbach, ΔP = f (L/D) (ρv²/2): 2 L/s of water through 100 m of 50 mm commercial steel pipe loses about 24.5 kPa (3.56 psi).
24.54 kPa
Velocity 1.019 m/s · Re 50736 (turbulent) · f = 0.02369 · head loss 2.507 m
| Unit | Value |
|---|---|
| Pa | 24539 |
| kPa | 24.539 |
| bar | 0.24539 |
| mbar | 245.39 |
| psi | 3.5591 |
| inH₂O | 98.516 |
| mmH₂O | 2502.3 |
- v = Q ÷ (π D² / 4) = 0.002 ÷ 0.0019635 = 1.01859 m/s
- Re = ρ v D / μ = 50736.4; ε/D = 0.0009
- f = Colebrook–White (iterated) = 0.0236944
- ΔP = f (L/D) (ρ v²/2) = 0.0236944 × 2000 × 517.831 = 24539.4 Pa
Darcy–Weisbach Formula
ΔP = f × (L / D) × (ρ v² / 2)
Colebrook–White: 1/√f = −2 log₁₀( ε/(3.7 D) + 2.51/(Re √f) ) · laminar: f = 64/Re
Head loss h = ΔP / (ρ g)
Worked Example
2 L/s of water at 20 °C through 100 m of 50 mm commercial steel pipe (ε = 0.045 mm):
- Velocity v = 0.002 ÷ (π × 0.05² / 4) = 1.019 m/s
- Reynolds number Re = 50736 (turbulent); ε/D = 0.0009
- Friction factor f = 0.02369 (Colebrook)
- ΔP = 0.02369 × 2000 × 517.8 Pa = 24.54 kPa = 3.559 psi; head loss 2.51 m
Pipe Roughness Values
| Material | ε (mm) | ε (in) |
|---|---|---|
| Drawn tubing, glass, PVC | 0.0015 | 0.0000591 in |
| Commercial steel | 0.045 | 0.00177 in |
| Galvanised steel | 0.15 | 0.00591 in |
| Cast iron | 0.26 | 0.0102 in |
| Concrete | 1 | 0.0394 in |
Roughness grows with age, scale and corrosion; use conservative values for old pipe.
Frequently asked questions
What is the Darcy–Weisbach equation?
ΔP = f (L/D) (ρv²/2): pressure drop equals the Darcy friction factor times the length-to-diameter ratio times the velocity pressure. It applies to any fluid in a full pipe.
How is the friction factor found?
For laminar flow (Re < 2300) f = 64/Re. For turbulent flow this calculator solves the Colebrook–White equation, which depends on Re and the relative roughness ε/D.
Does doubling the flow double the pressure drop?
No. In turbulent flow ΔP rises roughly with the square of flow. In the example below, halving 2 L/s to 1 L/s cuts the drop from 24.5 kPa to 6.87 kPa.
Does this include fittings and valves?
No, only straight pipe. Add fittings as equivalent length or with K-factors (ΔP = K ρv²/2).
Sources
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology .
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Pipe Pressure Drop Calculator. Retrieved October 17, 2026, from https://scientificconverters.com/calculators/pipe-pressure-drop/
IEEE
Scientific Converters, "Pipe Pressure Drop Calculator," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/calculators/pipe-pressure-drop/