Piping
Darcy-Weisbach Pressure Drop Calculator
The Darcy-Weisbach equation calculates friction pressure drop in a straight circular pipe for steady incompressible single-phase flow. The Darcy friction factor must be supplied by the user — this calculator does not compute, estimate, or look up friction factor. For friction factor estimation, use the separate Friction Factor calculator.
TypeInteractive calculator — separate from unit conversions
Calculator
Output units
Pressure:
Head:
Result
Pressure drop (ΔP)40 kPa
Head loss (h_f)4.07886 m
Dynamic pressure (ρv²/2)2 kPa
L/D ratio1000
Formulas
Pressure drop
ΔP = f × (L/D) × (ρv²/2)
Head loss
h_f = f × (L/D) × v²/(2g)
Dynamic pressure
q = ρv²/2
Length-to-diameter ratio
L/D = L / D
Diagram
Worked example
Water (ρ = 998 kg/m³) flows at 2.5 m/s through a 150 mm pipe, 100 m long. f = 0.020, g = 9.80665 m/s².
- 01L/D = 100 / 0.15 = 666.7
- 02Dynamic pressure = ρv²/2 = 998 × 2.5² / 2 = 3118.75 Pa
- 03ΔP = 0.020 × 666.7 × 3118.75 = 41,583 Pa ≈ 41.58 kPa
- 04h_f = 0.020 × 666.7 × 2.5²/(2 × 9.80665) = 4.249 m
Result
Pressure drop is 41,583 Pa (41.58 kPa). Head loss is 4.249 m.
FAQ
Does this use Darcy or Fanning friction factor?
This calculator uses the Darcy (Moody) friction factor exclusively. The Darcy friction factor is 4× the Fanning friction factor. Verify which convention your source uses before entering the value.
Is this a full line sizing tool?
No. This calculates friction pressure drop only. It does not size pipes, select pipe schedules, or include minor losses.
Does this calculate friction factor automatically?
No. You must supply the Darcy friction factor. Use the separate Friction Factor calculator if you need to estimate f from Reynolds number and roughness.
Does this include minor losses?
No. Minor losses from fittings and valves are not included. Use the Minor Loss calculator or Pipe Head Loss calculator to add minor losses.
Does this look up pipe roughness, viscosity, or pipe schedule?
No. All inputs are user-supplied. No hidden lookup tables are used.