How to Use the Exhaust Fan Airflow Calculator
When shopping for a bathroom or kitchen exhaust fan, the CFM (cubic feet per minute) rating on the box doesn't mean much on its own — you need to know how much airflow your specific room requires. This calculator multiplies your room's volume (cubic feet) by the recommended air changes per hour (ACH) for its use, then converts to CFM.
The formula is: required CFM = (length × width × height in ft³ × ACH) ÷ 60. For example, a 10ft × 7ft × 8ft bathroom (8 ACH) needs about 75 CFM. Rooms that generate more moisture or odor, like kitchens, need a higher ACH than living rooms or offices.
When picking an actual fan, it's smart to size up rather than match the number exactly. Duct length, elbows, and grille or filter resistance all reduce real-world airflow below the rated CFM, so a fan rated 10-20% above your calculated requirement performs closer to what you actually need. Also check the fan's noise rating (sones) and duct diameter before buying.
If your ceiling is unusually tall or the room has no windows for natural ventilation, consider sizing up one step further. If you're replacing an existing fan, confirm the new unit's duct outlet matches your current ducting so you can swap it in without rerouting anything.
Frequently Asked Questions
Typical recommendations are 8 ACH for bathrooms, 10 for laundry rooms, 15 for kitchens, 6 for living rooms/offices and garages, and 4 for basements. Rooms with more moisture or odor need a higher ACH.
Yes. Duct length, bends, and filter resistance usually reduce real-world airflow below the rated CFM, so choosing a fan rated 10-20% above your calculated requirement is a good practice.