Pressure Drop Calculator
Estimate line pressure loss from flow, diameter, length, and friction.
Calculator
No signup required. Results are indicative—verify for your standards.
Darcy-Weisbach style: ΔP = f × (L/D) × (ρv²/2). Flow rate in m³/s, diameter in mm.
Velocity5.09 m/s
Pressure drop77815 Pa (0.778 bar)
Formula
ΔP = f × (L/D) × (ρ × v² / 2), with v = Q/A, A = πD²/4. f is Darcy friction factor (typical 0.02–0.04 for turbulent steel pipe).
Example calculation
Water at Q = 0.05 m³/s through a 100 mm ID pipe, L = 100 m, f = 0.025: v = 6.37 m/s, ΔP = 0.025 × (100/0.1) × (1000 × 6.37²/2) ≈ 507,000 Pa ≈ 5.07 bar.
Engineering notes
Add fittings via equivalent length or K-factors. For laminar flow use Hagen–Poiseuille. Validate f with Moody chart or Colebrook for accuracy.
When to use this calculator
- Line sizing — confirm that a proposed pipe diameter gives acceptable pressure drop at design flow
- Pump head calculation — determine friction losses to include in total dynamic head calculation
- Control valve sizing — estimate available pressure differential across a control valve station
- Long-distance transfer lines — identify whether booster pumps are required for extended piping runs
- Process simulation — validate piping P&IDs before issuing for construction
Frequently asked questions
- What Darcy friction factor should I use for steel pipe?
- For fully turbulent flow in new commercial steel pipe (roughness ε ≈ 0.046 mm), the Darcy friction factor f is typically 0.018–0.025 for pipe diameters above 100 mm. Use the Colebrook-White equation or Moody chart for precision: 1/√f = −2 log₁₀(ε/(3.7D) + 2.51/(Re√f)). For initial estimates, f = 0.02 for turbulent flow in clean steel pipe is widely used.
- How do I account for pipe fittings (elbows, valves, tees)?
- The two standard methods are: (1) Equivalent length — look up the equivalent pipe length (L_eq) for each fitting from published tables (e.g. Crane TP-410) and add to the straight pipe length. (2) Resistance coefficient (K-factor) — add the K values for all fittings and multiply by ρv²/2 to get the additional pressure drop. For preliminary designs, add 20–30% to the straight-pipe loss as an allowance for fittings.
- When should I use Hagen-Poiseuille instead of Darcy-Weisbach?
- Hagen-Poiseuille applies only to laminar flow (Reynolds number Re < 2300). For laminar flow: ΔP = 128 × μ × Q × L / (π × D⁴), where μ is dynamic viscosity (Pa·s). Darcy-Weisbach applies to turbulent flow (Re > 4000), which covers most industrial piping. For transitional flow (2300 < Re < 4000), the flow is unstable and both formulas give approximate results.
- How do I convert pressure drop from Pa to bar or psi?
- 1 bar = 100,000 Pa. 1 psi = 6,894.76 Pa. So divide Pa by 100,000 for bar, or by 6,894.76 for psi. In process engineering, pressure drop in piping is often expressed in kPa or bar for liquid lines. For gas lines, it may be in mbar or mm WC for low-pressure systems.