Bathroom Fan CFM Calculator
Exhaust fan size for any bathroom.
Reviewed by the Calculation Machine team · How we check our maths
Your values
ⓘ Live results
Results
- Recommended fan
- 70 CFM
- By floor area rule
- 63 CFM
- By 8 ACH rule
- 67 CFM
- Room volume
- 504 cu ft
- Suggested duct
- 4 in
Quick answer
The standard rule is 1 CFM per square foot for bathrooms up to 100 sq ft, with a 50 CFM minimum, or 8 air changes per hour for larger rooms.
The formula
CFM = area x 1, or volume x 8 / 60 for large rooms
Worked example
Worked example
Using the values the calculator loads with.
Inputs
- Bathroom length9 ft
- Bathroom width7 ft
- Ceiling height8 ft
- Has jetted tub or steam shower?No
Result
- Recommended fan70 CFM
- By floor area rule63 CFM
- By 8 ACH rule67 CFM
- Room volume504 cu ft
- Suggested duct4 in
How to use the Bathroom Fan CFM Calculator
- 1Measure the bathroom in feet and enter length, width and height.
- 2Say whether there is a jetted tub or steam shower, which needs extra extraction.
- 3Buy the next fan size up and run it 20 minutes after showering.
Frequently asked questions
What CFM for a 5x8 bathroom?
40 sq ft needs the 50 CFM minimum; an 80 CFM fan clears steam faster and is barely louder.
Do I need a bigger duct?
A 50–80 CFM fan works on 4 inch duct; 100 CFM and above wants 6 inch to hit its rating.
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Report an error