Exhaust Fan Sizing Calculator Formula
Understand the math behind the exhaust fan sizing calculator. Each variable explained with a worked example.
Formulas Used
Fan Rating Needed
fan_cfm = room_vol * ach / 60Room Volume
room_volume = room_volCFM per Sq Ft Rule
fan_cfm_sqft = room_length * room_width * 1Variables
| Variable | Description | Default |
|---|---|---|
room_length | Room Length(ft) | 10 |
room_width | Room Width(ft) | 8 |
ceiling_height | Ceiling Height(ft) | 8 |
ach | Air Changes per Hour(ACH) | 8 |
room_vol | Derived value= room_length * room_width * ceiling_height | calculated |
How It Works
Exhaust Fan Sizing
CFM = Room Volume x ACH / 60
The HVI (Home Ventilating Institute) recommends a simpler rule for bathrooms: 1 CFM per square foot for rooms up to 100 sq ft, and 50 CFM minimum. For larger bathrooms, add 50 CFM per fixture (toilet, shower, tub). Kitchens need 100-400 CFM depending on cooking.
Worked Example
10 x 8 ft bathroom, 8 ft ceiling, 8 ACH.
- 01Volume = 10 x 8 x 8 = 640 cu ft
- 02CFM = 640 x 8 / 60 = 85 CFM
- 03Square foot rule = 80 CFM
- 04Use at least an 80-90 CFM fan.
Frequently Asked Questions
What sone rating is quiet?
Under 1.0 sone is considered very quiet (leaves rustling). 1-2 sones is quiet operation. Above 3 sones is noticeable. Look for fans rated under 1.5 sones for bathrooms.
Does duct length affect fan performance?
Yes. Longer duct runs and more elbows create back-pressure that reduces actual airflow. Check the fan performance curve at your estimated static pressure, typically 0.1 to 0.25 inches of water.
Ready to run the numbers?
Open Exhaust Fan Sizing Calculator