Bathroom Fan Size Calculator

Bathroom exhaust fan sizing uses the CFM (Cubic Feet per Minute) airflow rating to match fan capacity to room volume and ensure adequate ventilation. The standard is to replace the air in the bathroom at least 8 times per hour, though 10–12 air changes per hour (ACH) is recommended for moisture and odor control.

Basic CFM formula (by room volume): Required CFM = (Room Volume × ACH) ÷ 60

Where:

• Room Volume = Length × Width × Height (in feet) = cubic feet
• ACH = Air Changes per Hour (use 8 minimum, 10 recommended, 12 for steam showers)
• ÷ 60 converts from “per hour” to “per minute”

Simplified HVI rule (for bathrooms up to 100 sq ft): CFM = Square Footage × 1.1

• This approximates 10 ACH for an 8-foot ceiling

For bathrooms over 100 sq ft — add CFM per fixture:

• Toilet (separate compartment): +50 CFM
• Shower: +50 CFM
• Jetted tub: +100 CFM

CFM ratings and fan sizes:

• Small bath (< 50 sq ft): 50–80 CFM
• Standard bath (50–100 sq ft): 80–110 CFM
• Large bath (100–150 sq ft): 110–150 CFM
• Master bath with shower + tub: 150–300 CFM

Worked example: Bathroom: 9 ft × 7 ft × 8 ft ceiling. Has a separate shower.

• Room volume = 9 × 7 × 8 = 504 cubic feet
• Required CFM (10 ACH) = (504 × 10) ÷ 60 = 84 CFM
• Add shower fixture: 84 + 50 = 134 CFM

Select a fan rated at 150 CFM (the next standard size up). Undersizing a fan forces it to run longer with less effect; slightly oversizing is always preferable for moisture control and mold prevention.