Bathroom fan CFM = 1 CFM per square foot of floor area for bathrooms up to 100 sq ft, with a 50 CFM minimum, so an 8 × 10 ft bathroom (80 sq ft) needs an 80 CFM fan. Bigger bathrooms add up the fixtures instead. Enter your bathroom above to see how many CFM for a bathroom fan, including tall ceilings and continuous fans.
How many CFM for a bathroom fan
The Home Ventilating Institute (HVI) gives two rules, and the calculator applies the one that fits your room.
Under 100 sq ft: CFM = floor area (sq ft) × 1, never less than 50 CFM
Over 100 sq ft: CFM = toilet 50 + shower 50 + bathtub 50 + jetted tub 100 (for the fixtures you have)
HVI also allows a continuous fan: "Continuous ventilation at a minimum rate of 20 CFM may be used in lieu of an intermittent 50 CFM exhaust fan". ASHRAE 62.2, as reproduced by California's Energy Code Ace, lists the same 50 CFM on demand and 20 CFM continuous for a bathroom. Pick the "All the time" setting to see that case.
Worked examples
- Powder room, 5 × 6 ft = 30 sq ft: the area rule gives 30, but the minimum is 50 CFM.
- Full bath, 5 × 8 ft = 40 sq ft: 40, so 50 CFM.
- Bathroom, 8 × 10 ft = 80 sq ft: 80 CFM.
- Bathroom, 10 × 10 ft = 100 sq ft: 100 CFM.
- Master bath, 10 × 12 ft = 120 sq ft with toilet, shower and tub: 50 + 50 + 50 = 150 CFM. With a jetted tub instead of the plain tub: 50 + 50 + 100 = 200 CFM.
Bathroom fan size chart
| Bathroom size | Floor area | Rule | Fan size |
|---|---|---|---|
| 5 × 6 ft | 30 sq ft | minimum | 50 CFM |
| 5 × 8 ft | 40 sq ft | minimum | 50 CFM |
| 6 × 9 ft | 54 sq ft | 1 per sq ft | 54, buy 60 CFM |
| 8 × 10 ft | 80 sq ft | 1 per sq ft | 80 CFM |
| 10 × 10 ft | 100 sq ft | 1 per sq ft | 100 CFM |
| 10 × 12 ft | 120 sq ft | fixtures: toilet, shower, tub | 150 CFM |
| 10 × 12 ft, jetted tub | 120 sq ft | fixtures: toilet, shower, jetted tub | 200 CFM |
Fans come in steps, so the calculator rounds a fan size up to the next 10 CFM and you buy the next size at or above it.
Tall ceilings
HVI's 1 CFM per square foot assumes a normal ceiling. For a ceiling above 8 ft the calculator also works out the airflow for 6 air changes an hour, the low end of the 6 to 7 range Trane gives for bathrooms, and uses it when it is larger.
CFM = floor area × ceiling height × 6 ÷ 60
A 100 sq ft bathroom with a 10 ft ceiling comes to exactly 100 CFM, no change. At 12 ft it is 120 CFM, so the fan grows by 20. A small 5 × 6 ft powder room with a 10 ft ceiling needs 30 CFM by air changes, so the 50 CFM minimum still rules.
Duct, noise and switching
The rating on the box is the airflow at the fan; a long duct with elbows moves less. Check the duct diameter in the duct size calculator and keep the run short and vented to the outdoors. Airflow for the rest of the house is in the CFM calculator, and a kitchen hood has its own rules in the range hood CFM calculator.
When a fan is not enough
If a bathroom still fogs or grows mildew with a right-sized fan, the air is too damp for a fan alone. Compare the space with the dehumidifier size calculator, and if you are opening the walls anyway, price the job in the bathroom remodel cost calculator. To measure the room, use the square footage calculator. All the airflow tools are in the heating and plumbing calculators hub.