Calculator
Bathroom fan size calculator
Enter your room and fixtures to get the minimum exhaust fan airflow, measured in cubic feet per minute (CFM). The result updates as you type, and nothing leaves your device.
50 CFM minimum
Worked example (shown when scripts are off): an 8 ft by 5 ft bathroom with an 8 ft ceiling is 40 square feet. At 1 CFM per square foot that is 40 CFM, but 50 CFM is the usual floor, so you would pick a fan rated at 50 CFM or more.
Check before you buy
This is a minimum, not a guarantee. Local code and the fan maker's installation instructions come first, and a licensed electrician or HVAC pro can confirm what your house needs.
How the calculator works
The Home Ventilating Institute (HVI) recommends that a bathroom of up to 100 square feet gets 1 CFM of exhaust for each square foot of floor. That works out to roughly 8 air changes per hour in a room with an 8 ft ceiling. Multiply length by width, and that is your starting number.
For a room over 100 square feet, floor area stops being a good guide, so HVI adds up the fixtures instead: 50 CFM for each toilet, shower and bathtub, and 100 CFM for a whirlpool or jetted tub. The calculator rounds up to the next 10 CFM and never goes below 50 CFM, which is the intermittent exhaust minimum in the model residential code and in the ASHRAE 62.2 ventilation standard.
Worked example
Take a 12 ft by 10 ft bathroom, which is 120 square feet. It is over 100, so you add fixtures. A toilet (50), a shower (50) and a tub (50) give 150 CFM. A 5 ft by 8 ft hall bathroom is 40 square feet, so the area rule gives 40, and the 50 CFM floor applies.
Ceiling height
The 1 CFM per square foot rule assumes an 8 ft ceiling. One fan maker's support page gives a volume version for rooms under 100 square feet: length times width times ceiling height times 0.13, rounded up to the next 10. That is the same 8 air changes per hour idea applied to the actual volume. This calculator follows that logic, so for ceilings above 8 ft it scales the result by height divided by 8. For rooms over 100 square feet that scaling is our own extension, because we could not find a published HVI ceiling rule for the fixture method.
The calculator also never sizes a room over 100 square feet below the 100 square foot result. That is our guard, so a larger room cannot end up with a smaller fan than a smaller one.
Noise, ducting and venting
A fan you can stand to hear is a fan you use. Energy Star guidance suggests no more than 1 sone for fans that run for long periods and no more than 3 sones for short use. Lower sones means quieter, and quiet fans tend to get switched on and left on.
The fan must exhaust to the outdoors, never into an attic, crawl space or wall cavity. Use the duct size the fan maker specifies, keep the first 2 to 3 feet straight before any elbow, and keep the run short with few bends. Long, narrow or bent ducts cut the airflow well below the number on the box, so if your duct is long, choose a higher rated fan. HVI recommends running the fan about 20 minutes after you leave the bathroom, and some guidance says up to an hour after a shower. A timer switch makes that automatic.
When to call a professional
Hire a licensed electrician or HVAC pro if you need new wiring or a new circuit, if the existing duct is damaged or ends in an attic, if you are fitting a high-flow fan, or if the room is large or has a jetted tub. Switch the power off at the breaker before you touch a fan or its wiring.
Limits of this rule
These are minimums. They do not account for duct length, windowless rooms, very steamy daily use, or local code that may require more. The manufacturer's installation instructions and your local building code govern.
Where this comes from
We checked the Home Ventilating Institute bathroom ventilation page (1 CFM per square foot up to 100 square feet, fixture values above that, and the 20-minute run time), a fan maker's support page (the 0.13 volume factor), and the Pacific Northwest National Laboratory building science guide on bathroom exhaust fans (50 CFM intermittent and 20 CFM continuous minimums, straight duct run, sone guidance and outdoor termination). We did not find a published HVI ceiling adjustment for the fixture method, so that part is our own.