How Many CFM Does Your Bathroom Fan Need?
Part of our Ventilation & Hot Water guides.
An undersized fan leaves mirrors fogged and walls damp; an oversized one is noisy and wastes heated air. Sizing takes two minutes.
Step 1: bathrooms up to 100 sq ft
Use 1 CFM per square foot of floor, with a minimum of 50 CFM. Multiply length × width in feet.
| Bathroom size | Area | Fan |
|---|---|---|
| 5 × 8 ft | 40 sq ft | 50 CFM |
| 7 × 10 ft | 70 sq ft | 70-80 CFM |
| 8 × 10 ft | 80 sq ft | 80 CFM |
| 10 × 10 ft | 100 sq ft | 100-110 CFM |
Step 2: bathrooms over 100 sq ft
Add CFM per fixture: roughly 50 CFM each for a toilet, shower and standard tub, and 100 CFM for a jetted tub. A large master bath with toilet, shower and soaking tub would need about 150 CFM, often split into two fans.
Step 3: adjust for ceilings and ducts
For ceilings above 8 feet, size up. Long duct runs and many elbows reduce real airflow, so choose the next size up if the duct is long. Always vent outdoors, never into the attic.
Noise (sones)
Under 2 sones is considered quiet; 1.5 sones is typical for VEVOR's ceiling fans. A quiet fan matters because people turn off loud fans too soon.
Humidity sensor and timer
A fan should run about 20 minutes after a shower. A humidity-sensing fan or a timer switch does this automatically and is the single biggest improvement over a basic on/off fan.
Code requirements in brief
US model codes commonly require bathrooms to have either an openable window of a certain size or mechanical ventilation, often with minimum airflow such as 50 CFM intermittent or 20 CFM continuous. Local codes vary; your building department has the final word.
Continuous vs intermittent fans
Some modern homes use low-speed continuous ventilation with a boost mode. Most bathroom fans run intermittently: on during and after showers. Humidity-sensing fans bridge the two by switching on only when needed.
Where to place the fan
- Between the shower and the toilet, close to the shower.
- Away from the door so air sweeps across the room.
- Directly over a tub or shower only if the fan is rated for that location (often requires GFCI).
Common sizing mistakes
- Choosing by room size only and ignoring a long duct.
- Venting into the attic.
- Buying a loud fan that people switch off early.
Frequently asked questions
Is 80 CFM enough for a bathroom?
For bathrooms up to about 80 sq ft with standard ceilings, yes.
Is a bigger fan always better?
No. Oversized fans are louder and pull more conditioned air; match the fan to the room.