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.
room_length = 10room_width = 8ceiling_height = 8ach = 8
- 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.
Ready to run the numbers?
Open Exhaust Fan Sizing Calculator