Compute the Darcy-Weisbach major (friction) pressure drop in a straight pipe: ΔP = f·(L/D)·(ρ·v²/2) Pa and head loss h_f = f·(L/D)·v²/(2g) m. The user supplies the Darcy friction factor f (not the Fanning factor). Length in m/km/ft, diameter in m/cm/mm/inch, density in kg/m³/g/cm³/lb/ft³ — all normalised to SI internally. Returns ΔP in Pa, kPa and bar, and head loss in m and ft. Gravity defaults to 9.81 m/s² and may be overridden.
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Key facts
Category
Math & Numbers
Input types
number, select
Output type
json
Sample coverage
4
API ready
Yes
Overview
The Darcy-Weisbach pipe pressure drop calculator estimates major friction loss in a straight pipe using the Darcy friction factor, pipe length, internal diameter, fluid density, and mean velocity. It returns pressure drop in Pa, kPa, and bar, plus head loss in meters and feet.
When to use
Estimate friction pressure drop in a straight water, oil, or other fluid pipeline.
Compare how pipe length, diameter, density, or velocity affects head loss.
Convert a Darcy-Weisbach result into pressure and head-loss units for hydraulic calculations.
How it works
1Enter the Darcy friction factor f, pipe length, internal diameter, and mean flow velocity.
2Select supported units for length, diameter, and fluid density; values are normalized to SI internally.
3The calculator applies ΔP = f·(L/D)·(ρ·v²/2) and uses gravity to calculate head loss.
4Review pressure drop in Pa, kPa, and bar, along with head loss in meters and feet.
Use cases
Sizing and reviewing straight pipeline sections in hydraulic systems.
Estimating water or oil friction losses from known flow velocity and pipe geometry.
Checking pressure-drop and pump-head calculations using consistent engineering units.
Examples
1. Water pipeline friction loss
Hydraulic engineer
Background
An engineer is checking the friction loss in a 100 m water pipe with a 50 mm internal diameter and a mean velocity of 2 m/s.
Problem
Calculate the Darcy-Weisbach pressure drop and corresponding head loss for the straight pipe section.
How to use
Enter the Darcy friction factor, pipe length, diameter, water density, velocity, and default gravity.
f = 0.02; L = 100 m; D = 0.05 m; ρ = 1000 kg/m³; v = 2 m/s; g = 9.81 m/s²
Outcome
The calculator returns a pressure drop of 80,000 Pa, or 80 kPa and 0.8 bar, with a head loss of 8.1549 m or 26.7551 ft.
2. Oil pipeline pressure drop
Process engineer
Background
A process engineer needs to estimate friction loss for oil flowing through a 50 m pipe with a 0.1 m internal diameter.
Problem
Determine the pressure drop and head loss using the oil density, velocity, and Darcy friction factor.
How to use
Enter the oil density as 850 kg/m³, set the pipe dimensions and velocity, and use the default gravity value.
FAQ
What friction factor should I enter?
Enter the Darcy friction factor, also called the Moody or Darcy-Weisbach factor. Do not enter the Fanning friction factor.
What type of pressure loss does this calculator calculate?
It calculates major friction loss in a straight pipe. It does not calculate minor losses from fittings, valves, or bends.
Which length units are supported?
Pipe length can be entered in meters, kilometers, or feet.
Which diameter units are supported?
Internal pipe diameter can be entered in meters, centimeters, millimeters, or inches.
What is the default gravity value?
Gravity defaults to 9.81 m/s², and you can override it when needed.