Here’s a ready-to-paste HTML article on sizing a bathroom exhaust fan, with a CFM chart, code minimums, and a link to a 100 CFM fan roundup.
“`html
Most bathrooms need about 1 CFM of exhaust fan capacity per square foot of floor area, with a 50 CFM floor for rooms 50 square feet or smaller.
An undersized exhaust fan leaves the mirror fogged long after the shower ends and quietly pushes moisture into the drywall above it. An oversized one roars without clearing the room any faster. The number that settles both problems is CFM — cubic feet per minute — and for most U.S. bathrooms it comes straight off a tape measure.
Sizing a bathroom exhaust fan starts with floor area, and two things can override that figure: ceiling height and the plumbing fixtures in the room. The table below covers the common cases; the code minimums are the backstop.
How Many CFM Does A Bathroom Exhaust Fan Need?
For a bathroom between 50 and 100 square feet, plan on 1 CFM per square foot of floor area. Rooms of 50 square feet or less bottom out at 50 CFM, while rooms larger than 100 square feet switch to counting fixtures instead of floor space.
Floor area is a proxy for moisture load, which is why every sizing shortcut starts there. Shower volume, tub size, and how many people share the bathroom all push the actual figure higher, and none of them show up on a tape measure.
Measure the length and width in feet, multiply the two, and match the result to the table. A 7-by-10 bathroom lands at 70 square feet, so it calls for a 70 CFM fan.
| Bathroom Floor Area | Minimum Fan Rating | Example Room |
|---|---|---|
| Under 50 sq. ft. | 50 CFM | 5 x 8 bath (40 sq. ft.) |
| 50 sq. ft. | 50 CFM | 5 x 10 bath |
| 60 sq. ft. | 60 CFM | 6 x 10 bath |
| 70 sq. ft. | 70 CFM | 7 x 10 bath |
| 80 sq. ft. | 80 CFM | 8 x 10 bath |
| 100 sq. ft. | 100 CFM | 10 x 10 bath |
| Over 100 sq. ft. | Add 50 CFM per toilet, shower, or tub; 100 CFM for a jetted tub | 12 x 12 primary bath (144 sq. ft.) |
If the math lands near 100 CFM, a tested 100 CFM bathroom exhaust fan roundup saves you a second search — it compares airflow, sone ratings, and duct sizes side by side.
Sizing A Bathroom Exhaust Fan: Ceiling Height And Code Minimums
The 1 CFM per square foot rule assumes an 8-foot ceiling. Taller rooms hold more moisture-laden air, so scale the floor-area number up for height or switch to the volume method: room volume × 8 air changes per hour ÷ 60.
The Home Ventilating Institute’s bathroom exhaust sizing table is the reference most manufacturers print on their spec sheets, and its 1 CFM rule works out to roughly 8 air changes per hour at standard ceiling height. The Home Depot buying guide takes a similar path, multiplying length × width × height and then applying 0.13 to produce a minimum CFM rating.
Building code sets the floor. The International Residential Code, Section M1507, requires bathroom exhaust of 50 CFM intermittent or 20 CFM continuous, ducted straight outdoors.
Past 100 square feet, add fixtures rather than floor area: 50 CFM for each toilet, shower, or bathtub, and 100 CFM for a jetted tub.