What Size Bathroom Exhaust Fan Do I Need? CFM by Room Size
Updated October 23, 2026
For most bathrooms, you need an exhaust fan rated at 1 CFM (cubic foot per minute) for every square foot of floor, and never less than 50 CFM. A typical 5 × 8 foot bathroom needs a 50 CFM fan; a 9 × 10 foot bathroom needs 90 CFM. Bathrooms over 100 square feet are sized by their fixtures instead.
Bathrooms up to 100 square feet
The Home Ventilating Institute (HVI), which certifies fan ratings, recommends 1 CFM per square foot for bathrooms up to 100 square feet. With a standard 8-foot ceiling, that replaces the air about eight times an hour. Multiply the length by the width to get the area:
| Bathroom size | Floor area | Fan size |
|---|---|---|
| 5 × 8 ft | 40 sq ft | 50 CFM (code minimum) |
| 6 × 9 ft | 54 sq ft | 60 CFM |
| 8 × 10 ft | 80 sq ft | 80 CFM |
| 9 × 10 ft | 90 sq ft | 90 CFM |
| 10 × 10 ft | 100 sq ft | 100 CFM |
If your ceiling is taller than 8 feet, the room holds more air, so a fan sized only by floor area changes the air less often. The calculator scales the fan up in proportion, for example by 25% for a 10-foot ceiling, to keep the same rate. That adjustment is this site's rule of thumb, not part of HVI's guidance.
Bathrooms over 100 square feet
For larger bathrooms, HVI adds up the fixtures instead:
| Fixture | Ventilation |
|---|---|
| Toilet | 50 CFM |
| Shower | 50 CFM |
| Bathtub | 50 CFM |
| Jetted tub | 100 CFM |
A large bathroom with a tub, a separate shower, and a toilet needs 150 CFM. HVI notes you can provide it with one 150 CFM fan or with smaller fans over each fixture, which clears moisture where it starts. An enclosed toilet room needs its own fan or an opening window.
The building-code minimum
The International Residential Code, which most U.S. building codes are based on, requires a bathroom fan to move at least 50 CFM if it runs when needed, or 20 CFM if it runs continuously. The air must go directly outdoors, not into the attic or a crawl space. The code also says the fan has to deliver its rating against some resistance from the duct (0.25 inch of water column), which is why the duct run matters too.
Ducting and other tips
- Keep the duct short and straight. Every foot of duct and every bend reduces the air a fan actually moves. With a long run, choose a bigger fan.
- Put the fan near the shower or tub, where the steam is, and away from where fresh air comes in, so the air is pulled across the room.
- Leave a gap under the door. HVI recommends undercutting the bathroom door so replacement air can get in.
- Run it for 20 minutes after a shower. HVI recommends this, and a timer switch makes it automatic.
- Choose an HVI-certified fan, so its CFM and noise ratings have been tested.
A new bathroom fan costs about $250 to $950 installed, according to HomeGuide; replacing an existing fan with the same duct is at the low end.
Sources
Open the Bathroom Fan Size Calculator →
More guides
Figures are estimates based on published national data and are not a quote. Actual costs vary by region, contractor, and materials.