Construction methodology of eracting rectangular hvac duct

Thank you for posting your query.

Please refer - ANSI/SMACNA 006-2006, HVAC Duct Construction Standards.

To calculate CFM, you need to know the size of your heating or cooling unit in tons. Multiply this number by 400, which is the average output of an HVAC unit. Then divide by the total square footage of your home. This will give you the multiplier for all your rooms' CFM