Duct Velocity Calculator

Why duct velocity matters

Airflow volume (CFM) alone does not tell you whether a duct is correctly sized - the same CFM pushed through a smaller duct moves much faster, which increases both noise and pressure loss along the run.

Worked example

For 800 CFM through a duct with 1 square foot of cross-sectional area:

Velocity = 800 / 1 = 800 ft/min

Duct Section	Recommended Velocity
Main trunk line	600 - 900 FPM
Branch duct	500 - 700 FPM
Return air duct	400 - 600 FPM

The 800 FPM result above sits right at the upper edge of a typical main trunk range - comfortable for a trunk line, but likely too fast and noisy if this were a branch duct feeding a single room register.