Bathroom Fan CFM Calculator
Exhaust fan size for any bathroom.
Enter your values
Enter the bathroom length in ft.
Enter the bathroom width in ft.
Enter the ceiling height in ft.
Choose one of: No, Yes.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
CFM = area x 1, or volume x 8 / 60 for large rooms
At a glance
| What it does | Exhaust fan size for any bathroom. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Bathroom length, Bathroom width, Ceiling height, Has jetted tub or steam shower? |
| Main output | Recommended fan |
| Formula | CFM = area x 1, or volume x 8 / 60 for large rooms |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Bathroom length(ft)
- Enter the bathroom length in ft.
- Bathroom width(ft)
- Enter the bathroom width in ft.
- Ceiling height(ft)
- Enter the ceiling height in ft.
- Has jetted tub or steam shower?
- Choose one of: No, Yes.
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
Results
- Recommended fan70 CFM
- By floor area rule63 CFM
- By 8 ACH rule67 CFM
- Room volume504 cu ft
- Suggested duct4 in
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.
What do I need to enter into the Bathroom Fan CFM Calculator?
Just 4 values: bathroom length, bathroom width, ceiling height, has jetted tub or steam shower?. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Bathroom Fan CFM Calculator work out the 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. It applies the formula CFM = area x 1, or volume x 8 / 60 for large rooms and shows the working so you can check each step by hand.
Is the Bathroom Fan CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.