One r/HomeImprovement post describes it perfectly: an 80 CFM fan, freshly cleaned, in a 50 sq ft bathroom, and after a 25-minute hot shower the mirror is still completely fogged. Someone else upgraded from 50 to 150 CFM and said it was "better," but not fixed.
Here's the thing: on paper, an 80 CFM fan is plenty for that room. When a correctly sized fan doesn't work, the cause is almost always airflow, not fan size.
Short answer: a bathroom needs about 1 CFM per square foot of floor, with a 50 CFM minimum. For bathrooms over 100 sq ft, add about 50 CFM per toilet, shower or tub. If your fan meets that and the mirror still fogs, check the duct, the run time and whether air can get into the room.
Sizing the fan
| Bathroom | Floor area | Recommended fan |
|---|---|---|
| 5 × 7 ft | 35 sq ft | 50 CFM |
| 5 × 8 ft | 40 sq ft | 50 CFM |
| 6 × 8 ft | 48 sq ft | 70 CFM |
| 8 × 10 ft | 80 sq ft | 90 CFM |
| 10 × 12 ft | 120 sq ft | 130 CFM or more |
These come from our Bathroom Fan CFM Calculator. It takes the larger of the area method and a volume method (8 air changes an hour), so tall ceilings get a bigger fan.
Why the mirror still fogs
1. The duct is choking it
A fan's CFM rating is measured with almost no duct attached. A long run of crushed flexible duct, too many bends, or a 3-inch duct on a fan built for 4 inches can cut the real airflow in half. This is the most common fix in Reddit threads.
- Use rigid or semi-rigid duct, as short and straight as possible.
- Match the duct to the fan's outlet. Go up to 6 inches for fans over about 110 CFM.
- Vent fully outdoors, through the roof or wall. Never into the attic, where it causes mold.
2. It's not running long enough
Fans need to run during the shower and about 20 minutes after. Switching it off when you step out leaves most of the moisture in the room. A timer switch or a humidity-sensing switch solves this permanently.
3. Air can't get in
A fan can only pull out as much air as can get into the room. If the door seals tight, the fan just spins. You want a gap of about ¾ inch under the door. One r/homeowners thread found the opposite problem: in a drafty house, the fan pulled air from everywhere except the bathroom.
4. The backdraft flap is stuck
A damper that won't open fully (or a roof cap blocked by a bird's nest) quietly kills airflow. Hold a tissue up to the grille. If it barely moves, look at the damper and the outside vent.
5. It's in the wrong spot
Ideally the fan sits near or above the shower, so it catches steam before it spreads. Moving a fan is a bigger job. Fix the first four before you consider it.
Is a bigger fan ever the answer?
Yes, when the room really is undersized: tall ceilings, a large shower, a long hot shower routine, or a fan that was cheap to begin with (many builder-grade fans are 50 CFM in rooms that need 80). Look for a quiet fan rated 1.0 sone or less so you'll actually run it.
A cheap test first
Before buying anything, run the shower, prop the door open 2 inches, and run the fan for 20 minutes after. If the mirror clears much faster, you've found the problem: makeup air and run time. A $20 timer switch and a door undercut may be all you need.
Planning a full bathroom refresh? See small bathroom ideas, or upload a photo to the AI room designer to try new tile and vanity looks first.
Frequently asked questions
What CFM bathroom fan do I need?
About 1 CFM per square foot of floor space, with a 50 CFM minimum. A 6 × 8 ft bathroom needs about 70 CFM. Add 50 CFM per fixture for bathrooms over 100 sq ft.
Why does my bathroom mirror fog up even with the fan on?
Usually because of a restricted or too-long duct, too little run time after the shower, or no gap under the door to let air in.
How long should a bathroom fan run after a shower?
About 20 minutes. A timer or humidity-sensing switch makes this automatic.
Can a bathroom fan be too powerful?
Rarely for comfort, but a very large fan in a tight house can pull air from other vents. Size it with the 1 CFM per square foot rule and fix the ducting first.