Bathroom Vent Fan Calculator
Size your bathroom exhaust fan correctly. Enter your bathroom dimensions to get the minimum CFM rating per building code, recommended sone level, and duct size. Proper ventilation prevents mold, moisture damage, and mirror fogging.
Enter Bathroom Details
Full Breakdown
Floor Area
Volume
Method
Minimum CFM
Recommended CFM
Duct Size
Noise Level
Air Changes/Hour
How to Use This Calculator
- Enter the bathroom length, width, and ceiling height.
- Specify whether the room has a shower, tub, or jetted tub.
- Note if there are enclosed toilet or shower areas (they may need separate fans).
- Click "Calculate Fan Size" for CFM and duct recommendations.
- Building code requires a minimum of 50 CFM for bathrooms under 100 sq ft, or 1 CFM/sq ft for larger bathrooms.
The Formula
Small bathrooms (≤100 sq ft): Minimum 50 CFM
Large bathrooms (>100 sq ft): 1 CFM per sq ft
Air Changes per Hour = (CFM × 60) ÷ Room Volume
Per ASHRAE 62.2 and IRC M1507.4, bathrooms require continuous ventilation or intermittent exhaust. The HVI recommends 8 air changes per hour (ACH) for bathrooms. Jetted tubs and enclosed areas warrant additional CFM. A 4" duct handles up to 100 CFM; use 6" duct for higher airflow.