CalcCoverageHow bathroom fans are sized — the HVI method
Exhaust fan sizing follows the Home Ventilating Institute (HVI) guidance, and it splits bathrooms in two. Up to 100 sq ft, the rule is 1 CFM per square foot of floor — that is 8 air changes an hour at a standard 8 ft ceiling. Above 100 sq ft, the area stops telling the story and the fixtures take over: 50 CFM for each toilet, each shower and each tub, and 100 CFM for a jetted tub, because each fixture is its own source of moisture.
Why the duct setting exists
A fan's CFM rating is measured in free air, with nothing attached. Real duct runs push back: every elbow and every foot of corrugated flex duct steals airflow, and a long flex run can cost a fan a quarter of its rating. The duct setting adds the standard planning margin (about 15% for a typical run, 30% for a long or flexible one) so the fan you buy delivers the airflow the room needs at the end of its duct, not at the bench.
| Bathroom | Basis | Need | Standard fan |
|---|---|---|---|
| 5 × 8 ft powder room (toilet, sink) | 40 sq ft area | 40 CFM | 50 CFM |
| 8 × 6 ft family bath | 48 sq ft area | 48 CFM | 50 CFM |
| 10 × 8 ft bath, 9 ft ceiling | 80 sq ft × 9/8 | 90 CFM | 90–100 CFM |
| 12 × 10 ft with tub + shower + toilet | 3 fixtures × 50 | 150 CFM | 150 CFM |
Two buying notes the CFM number does not carry: sones are the noise scale, and 1.0 sone or under is a fan you can hold a conversation over — cheap loud fans get switched off, and an off fan moves no moisture. And duct diameter matters as much as fan size: a 110 CFM fan choked onto a long 4 in duct will not move its rating; 6 in duct is the honest partner for fans above about 100 CFM.