HVAC Duct Sizing & CFM Calculator
Calculate the required airflow (CFM) for a room using area, ceiling height, and air changes per hour. Then size the supply duct diameter based on a target air velocity.
Results
What is it?
CFM (Cubic Feet per Minute) is the fundamental airflow unit in HVAC design. This calculator uses the ACH method to determine how much air a room needs, then back-calculates the duct diameter required to deliver that air at a comfortable velocity. Duct velocity directly impacts noise, energy efficiency, and comfort.
How to use
Enter the room floor area, ceiling height, and select the appropriate ACH rate for the space type. Then enter your target duct velocity � lower velocities are quieter. The calculator shows total CFM needed and the ideal round duct diameter, rounded up to the nearest standard 2-inch increment.
Example scenario
A 400 ft� office with 9 ft ceilings (3,600 ft�) at 8 ACH needs 480 CFM. At a duct velocity of 700 FPM, the duct area is 0.686 ft�, requiring a 11.2-inch duct � round up to 12-inch standard duct for the supply run.
Pro tip
The equal friction method is preferred over ACH for larger systems � target 0.08�0.1 in.wg per 100 ft for main trunks. For branch ducts, reduce velocity to 400�500 FPM to prevent noise complaints at supply registers. Always add 10�15% to CFM calculated by ACH to account for duct leakage.