Friction Rate Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026Standardizing pressure loss for duct design
Duct friction charts and sizing tables express pressure loss as a rate per 100 feet of duct, which lets designers compare pressure loss across duct runs of different lengths on a level playing field.
Worked example
For a duct run losing 0.08 inches of water gauge over 80 feet:
Friction Rate = (0.08 / 80) x 100 = 0.1 in. w.g. per 100 ft
Frequently asked questions
What friction rate do residential systems typically target? Most residential duct systems are designed around 0.08 to 0.10 in. w.g. per 100 ft, balancing acceptable airflow noise against reasonable duct sizing.
How is this used in practice? Once a target friction rate is chosen, a duct friction chart (cross-referencing airflow CFM against friction rate) gives the correct duct diameter needed for each branch and trunk in the system.