Fans are rated in CFM, the volume of air they move each minute. The Home Ventilating Institute, which tests and certifies fans, publishes simple rules for bathrooms and kitchens, and the model building code sets a minimum for each. For any other room, airflow follows from its volume and how many times an hour you want the air replaced.
How to use this calculator
- Choose bathroom fan, range hood or any room.
- For a bathroom, enter the floor size and count the toilets, showers and tubs: large bathrooms are sized by fixture.
- For a range hood, enter the width of the cooktop and whether the hood hangs against a wall or over an island.
- For any room, enter its size and ceiling height and the air changes per hour you want.
Worked examples
A small bathroom
What are you sizing?: Bathroom exhaust fan · Room: 8 × 5 ft
- Bathroom fan
- 50 cfm
- Floor area
- 40 sq ft
A large main bathroom
What are you sizing?: Bathroom exhaust fan · Room: 14 × 10 ft · Toilets: 1 · Showers: 1 · Bathtubs: 1
- Bathroom fan
- 150 cfm
- Floor area
- 140 sq ft
- Code minimum for a fan run when needed
- 50 cfm
A range hood for a wall cooktop
What are you sizing?: Kitchen range hood · Cooktop width: 30 in · Hood mounted: Against a wall
- Range hood airflow
- 250 cfm
- HVI minimum
- 100 cfm
An island range hood
What are you sizing?: Kitchen range hood · Cooktop width: 36 in · Hood mounted: Over an island
- Range hood airflow
- 450 cfm
- HVI minimum
- 150 cfm
- Code minimum for a kitchen fan run when needed
- 100 cfm
A workshop by air changes
What are you sizing?: Any room, by air changes · Room: 20 × 16 ft · Ceiling height: 9 ft · Air changes per hour: 6
- Airflow
- 288 cfm
- Room volume
- 2,880 cubic feet
Bathroom exhaust fans
For a bathroom up to 100 square feet, HVI recommends 1 cfm per square foot of floor, which is about 8 air changes per hour for an ordinary ceiling, and at least 50 cfm for the smallest rooms.
Larger bathrooms are sized by what is in them: 50 cfm for each toilet, 50 cfm for each shower, 50 cfm for each tub and 100 cfm for a jetted tub. The model code's minimum for a bathroom fan that runs when needed is 50 cfm, or 20 cfm for one that runs all the time.
Range hoods
HVI recommends 100 cfm per foot of cooktop of width for a hood against a wall, and no less than 40 cfm per foot of cooktop. An island hood has no wall to guide the smoke, so it needs more: 150 cfm per foot of cooktop, with 50 cfm per foot of cooktop as the least.
The model code asks for at least 100 cfm from a kitchen fan that runs when needed. Very large hoods in tight houses can need make-up air, so check your local code before you buy one.
Ducts change what you get
- A fan's rating is measured against a set resistance. Long ducts, flex duct and elbows cut the real airflow, so pick a fan above the number you need.
- Use the duct size the fan calls for, and run it to the outside, never into the attic.
- A fan that runs on a timer or humidity switch clears moisture after the shower ends.
Common questions
How many CFM do I need for a bathroom fan?
For a bathroom up to 100 square feet, about 1 cfm per square foot of floor, with at least 50 cfm. Larger bathrooms are sized by their fixtures. Enter your bathroom above to get the number.
How many CFM do I need for a range hood?
HVI recommends 100 cfm per foot of cooktop of cooktop width for a hood against a wall and 150 cfm per foot of cooktop for one over an island.
How do I calculate CFM for a room?
Multiply the length, width and ceiling height to get the volume in cubic feet, multiply by the air changes per hour you want, and divide by sixty minutes.
What is the code minimum for a bathroom fan?
The model residential code asks for 50 cfm from a fan that runs when needed, or 20 cfm from one that runs all the time. Local codes can ask for more.