Enter a room's length, width, and height, then pick a room type or custom air-changes-per-hour rate to calculate required airflow in CFM.
How to Use the CFM Calculator
Enter the room's length, width, and ceiling height, then either pick a room type from the dropdown or type a custom air-changes-per-hour value directly into the number field next to it — both control the same setting, so picking a preset just fills in a starting number you're free to override.
Airflow in cubic feet per minute comes from two things: how much air the room holds, and how many times per hour that air needs to be replaced.
room volume (ft³) = length × width × height
CFM = room volume × air changes per hour ÷ 60The ÷ 60 converts an hourly rate into a per-minute one, since CFM is, by definition, a per-minute figure. A kitchen's 15 ACH default sits far above a bedroom's 4 — cooking generates heat, moisture, and odor fast enough that the air needs replacing roughly every four minutes, versus every fifteen for a room where nobody's doing much more than sleeping.
This tool stops at required airflow. Actually hitting that number with a real fan or duct run means accounting for static pressure losses from ductwork, filters, and bends — a fan rated for 400 CFM in open air can move noticeably less once it's pushing air through six feet of flex duct and a filter. Sizing the actual equipment is a separate step this calculator doesn't cover.