What is the Darcy-Weisbach equation?

hf = f × (L/D) × (v²/2g), where hf is head loss (m or ft), f is the Darcy friction factor (dimensionless), L is pipe length, D is inner diameter, v is average flow velocity, and g is gravitational acceleration (9.81 m/s² or 32.174 ft/s²).

What is the Darcy friction factor?

A dimensionless value that captures the effect of pipe roughness and flow regime on friction. For laminar flow (Re < 2300), f = 64/Re. For turbulent flow, use the Moody chart or Colebrook-White equation. Smooth commercial steel pipes typically have f ≈ 0.010–0.020.

How do I convert head loss to pressure drop?

Pressure drop ΔP = ρ × g × hf, where ρ is fluid density (1000 kg/m³ for water) and g is 9.81 m/s². This calculator performs that conversion automatically.

What is the difference between Darcy and Fanning friction factors?

The Darcy (Moody) friction factor is 4 times the Fanning friction factor. This calculator uses the Darcy friction factor, which is more common in civil and mechanical engineering. Make sure the value you enter matches this convention.

What Reynolds number indicates turbulent flow?

Re > 4000 is fully turbulent; Re < 2300 is laminar; 2300–4000 is the transition zone. The calculator computes Re from your inputs so you can confirm the flow regime before choosing a friction factor.

Darcy-Weisbach Calculator

Calculate pressure drop and head loss in pipe flow using the Darcy-Weisbach equation.

Pipe Parameters

Unit System

Pipe Length (m)

Inner Diameter (m)

Flow Velocity (m/s)

Darcy Friction Factor (f, dimensionless)

Laminar: f = 64/Re — Turbulent: use Moody chart

Fluid Density (kg/m³)

Darcy-Weisbach Equation

h_f = f × (L / D) × (v² / 2g)

h_f = head loss (m or ft)

f = Darcy friction factor

L = pipe length

D = inner diameter

v = flow velocity

g = gravity (9.81 m/s²)

Reynolds Number Helper

Dynamic Viscosity (Pa·s)

Summary

The Darcy-Weisbach equation is the standard method for calculating frictional head loss in pipes and ducts. It relates head loss to pipe geometry (length and diameter), flow velocity, and the dimensionless Darcy friction factor. This calculator handles water and common fluids in both SI (meters/Pa) and imperial (feet/psi) unit systems.

How it works

Enter pipe length, inner diameter, and flow velocity.
Choose your unit system (SI or imperial).
Enter the Darcy friction factor — use the Moody chart or the Colebrook equation for your Reynolds number and relative roughness.
Click Calculate to get head loss (hf) and pressure drop (ΔP).
Use the Reynolds number helper to estimate the flow regime (laminar or turbulent).

Use cases

Size pumps by computing the friction head they must overcome.
Verify pressure drop in HVAC duct systems.
Design water distribution networks and irrigation pipelines.
Check flow feasibility in chemical plant piping layouts.
Estimate energy losses in hydraulic power systems.
Compare pressure drops for different pipe diameters or materials.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu