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Mechanical Engineering2D

Darcy-Weisbach Pressure Drop Calculator

Calculate pressure drop in pipes using the Darcy-Weisbach equation with full parameter control.

Inputs
Enter your values below

0.015-0.03 typical for turbulent flow

Water = 1000, Air = 1.2

Result
Pressure Drop
—
Enter values and click Calculate
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Formula
friction × length ÷ diameter × density × velocity² ÷ 2 ÷ 1000
Result unit: kPa

ΔP = f × (L/D) × (ρv²/2). Friction factor f (dimensionless), L/D in m, ρ in kg/m³, v in m/s; /1000 for kPa.

Introduction

The Darcy-Weisbach equation is the most accurate method for calculating pressure drop in pipes for all flow regimes. Requires an estimated Darcy friction factor (Moody chart or Colebrook equation).

How This Calculator Works

ΔP = f × (L/D) × (ρv²/2). Friction factor f (dimensionless), L/D in m, ρ in kg/m³, v in m/s; /1000 for kPa.

Step-by-step process:

  1. Enter your input values in the calculator above
  2. The engine converts all inputs to SI base units (meters, kg, Pa)
  3. The formula is evaluated: friction * length / diameter * density * velocity^2 / 2 / 1000
  4. Result is formatted with the appropriate unit and precision

Calculation Example

100m of 100mm pipe, f=0.02, water at 2m/s: ΔP = 40 kPa.

Inputs: friction=0.02, length=100, diameter=100, density=1000, velocity=2
Result: 40 kPa

Unit Conversion for Darcy-Weisbach

**Pressure units**

- 1 bar = 100 kPa = 14.5038 psi

- 1 psi = 6.8948 kPa = 0.06895 bar

- 1 m water head ≈ 9.81 kPa ≈ 0.0981 bar

**Length units**

- 1 m = 3.28084 ft

- 1 inch = 25.4 mm

**Flow units**

- 1 m³/h = 0.2778 L/s = 4.403 US gpm

The Darcy-Weisbach equation ΔP = f × (L/D) × (ρV²/2) requires consistent SI units: length in m, velocity in m/s, density in kg/m³, giving pressure drop in Pa. Convert to bar or psi afterwards.

Engineering Applications

  • •Pipeline design
  • •Pump head calculation
  • •HVAC hydronic design
  • •Process piping

Frequently Asked Questions

How do I find the friction factor?

Use the Moody chart or solve the Colebrook-White equation. For fully turbulent flow in commercial steel pipe, f ≈ 0.015-0.022.

When should I use Darcy-Weisbach vs Hazen-Williams?

Darcy-Weisbach works for all fluids and flow regimes. Hazen-Williams is only for water at typical temperatures and is simpler but less accurate.

What is the Swamee-Jain equation?

The Swamee-Jain equation is an explicit formula for the Darcy friction factor: f = 0.25 / [log10(epsilon/(3.7D) + 5.74/Re^0.9)]^2. It approximates the Colebrook-White equation within 1% accuracy for Re > 5000 and is widely used in pipe flow calculations. Use this calculator for instant friction factor and pressure drop results.

How do I convert pressure drop between bar and psi?

1 bar = 14.5038 psi = 100 kPa. For water, 1 m head ≈ 0.0981 bar. Convert using these factors when comparing results with pump curves or valve datasheets.

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Engineering Disclaimer: Calculations are for reference and educational purposes only. Always verify results independently for engineering design. See full disclaimer.
Reviewed by: Industrial Engineering Team
References: ASME B31.3, ASTM A36, Perry's Chemical Engineers' Handbook