Darcy-Weisbach Equation: Flow Resistance & Pressure Loss Calculator

Calculate pressure loss and head loss in pipes, tubes, and ducts using the Darcy-Weisbach equation. Supports both velocity-based and volume flow calculations with SI and Imperial units.

Calculator

Inputs

Results

Pressure Loss-- Pa
Calculator

Inputs

Results

Head Loss (Water Column)-- m H₂O
Head Loss (mm H₂O)-- mm H₂O
ParameterSymbolSI UnitValue (SI)Imperial UnitValue (Imperial)
Density of Air (20°C)ρfkg/m³1.2slugs/ft³0.0237
Density of Waterρwkg/m³1000lb/ft³62.425
Specific Weight of WaterγwN/m³9807lbf/ft³62.4
Acceleration of Gravitygm/s²9.81ft/s²32.174
Darcy-Weisbach Friction Coefficientλ-0.015-0.08-0.015-0.08

Darcy-Weisbach Equations

Pressure Loss (Major Loss)

Δp_major_loss = λ (l / d_h) (ρ_f v² / 2)

Where:

  • Δp_major_loss = major (friction) pressure loss (Pa, psf)
  • λ = Darcy-Weisbach friction coefficient
  • l = length of duct or pipe (m, ft)
  • d_h = hydraulic diameter (m, ft)
  • ρ_f = density of fluid (kg/m³, slugs/ft³)
  • v = velocity of fluid (m/s, ft/s)

Head Loss (Water Column Reference)

Δh_major_loss,w = λ (l / d_h) (ρ_f / ρ_w) (v² / (2g))

Where:

  • Δh_major_loss,w = major head loss (m H₂O, ft H₂O)
  • g = acceleration of gravity (9.81 m/s², 32.174 ft/s²)
  • ρ_w = density of water (1000 kg/m³, 62.425 lb/ft³)

Head Loss in mm H₂O (Metric)

Δh_major_loss,w (mmH₂O) = λ (l / d_h) (ρ_f / ρ_w) (v² / (2g)) / 1000

Head Loss in Inches H₂O (Imperial)

Δh_major_loss,w (inH₂O) = 12 × λ (l / d_h) (ρ_f / ρ_w) (v² / (2g))