🌀CFM Calculator
Size an exhaust fan the easy way: room size plus how many times an hour the air should change.
Your room
You need about
51 CFM
Recommended fan: 70 CFM · 384 cu ft × 8 ACH ÷ 60
room cu ft
384
air changes/hr
8
CFM needed
51
fan size
70 CFM
Long or bendy duct runs reduce real airflow. If your duct is longer than about 25 feet, step up one fan size.
How it works
CFM = room volume (cu ft) × air changes per hour ÷ 60
Default air changes per hour (ACH): bathroom 8, kitchen 15, bedroom 6, garage 6, workshop 10. We then round up to the next common fan size.
Worked example
An 8 × 6 ft bathroom with an 8 ft ceiling:
- 8 × 6 × 8 = 384 cu ft
- 384 × 8 ÷ 60 = 51 CFM → a 70 CFM fan
Tips
Long duct runs, elbows and small ducts all reduce real airflow, so it is wise to size up one step.
Questions people ask
Cubic feet per minute — how much air a fan moves.
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Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.