Duct design
Duct Friction Loss Calculator
Estimate straight round-duct friction loss with transparent input assumptions.
Calculator
How to use
Provide a verified project friction factor and air density. This calculator covers straight round duct only; it does not add fittings, transitions, dampers, or equipment losses.
Formula
Variables
f is Darcy friction factor, L is length, D is diameter, ρ is density, and V is velocity. Units are converted internally to SI before reporting inches of water gauge.
Example
Enter the duct's CFM, diameter, straight length, measured or specified density, and friction factor from the selected design method.
Result interpretation
Do not treat this as total external static pressure. Add fitting losses and confirm the complete system with an appropriate duct design method.
FAQ
Why must I enter a friction factor?
Friction factor varies with duct surface, Reynolds number, and conditions; making it explicit avoids presenting a universal value as a design standard.