Straight-duct pressure check
Duct Friction Loss Calculator
Estimate friction loss for a straight, round duct using explicit Darcy-Weisbach inputs.
Calculate straight-duct loss
Use air density and Darcy friction factor from the method and conditions applicable to your project.
How to use
- Enter the CFM, inside diameter, and straight length for one round duct section.
- Enter the air density and Darcy friction factor from your stated conditions and selected method.
- Use the output only as the friction contribution for that straight section.
Formula
f is Darcy friction factor, L is straight length, D is inside diameter, ρ is density, and V is mean air velocity. The calculator converts the inputs to SI internally and reports the pressure result in inches of water gauge.
Why the friction factor is an input
Friction factor varies with the selected equation convention, surface condition, Reynolds number, geometry, and air conditions. Requiring it as an input prevents the calculator from presenting an unsupported universal duct-design default.
Example
Enter the verified CFM, inside diameter, straight length, density, and Darcy friction factor for the section being checked. The result message also reports the calculated velocity and pressure loss in Pa for review.
Result interpretation
This result is not total external static pressure and not a universal friction-rate recommendation. Add transitions, elbows, takeoffs, dampers, terminals, filters, coils, equipment, and other components with an appropriate complete-system design method.
Check the complete air path
Use the Duct Velocity Calculator to verify geometry and the Duct Friction Loss Explained guide for the difference between a straight-run calculation and total system resistance.
Frequently asked questions
Why must I enter a friction factor?
It depends on duct surface, flow regime, air conditions, and the selected method. A single default could imply an unsupported design standard.
Can I use this for rectangular duct?
No. This calculator uses the round-duct area and diameter relationship. For a rectangular comparison, use a documented equivalent-diameter method and verify the complete design separately.
Does the result include elbows or fittings?
No. It covers only the stated straight round-duct length. Fittings and components require additional loss calculations.