Small ducts mean noise and restriction; big ducts waste space and money. Enter the airflow you need and the calculator gives the round duct size and rectangular options.
Velocity drives noise. 700 fpm residential keeps airflow comfortable and whisper-quiet.
Duct area = CFM / velocity — the classic sizing approach for branch ducts.
Get both a round diameter and a rectangular equivalent for tight joist bays.
All calculations run locally in your browser.
Comfort through correct sizing.