How to Choose a Bathroom Window Exhaust Fan: Sizing and Venting Guide

Match fan size to your bathroom’s square footage at 1 CFM per square foot, with a 50 CFM floor, and always vent the exhaust outdoors.

A window-style fan swaps into an existing window opening instead of cutting a new ceiling hole, which makes duct routing the decision that matters most. Get the airflow right and the vent path right, and the rest of the choices fall into place quickly.

How Much Airflow Does Your Bathroom Need?

For bathrooms up to 100 square feet, size the fan at 1 CFM per square foot with a minimum of 50 CFM. Measure length times width, then round up to the nearest available size.

A 5-by-8 bathroom is 40 square feet, so the 50 CFM floor applies. A 9-by-10 bathroom is 90 square feet and needs at least 90 CFM.

• Up to 100 square feet: square footage in CFM, never below 50.
• Over 100 square feet: size by fixtures instead of floor area. A common approach assigns roughly 50 CFM to a toilet, shower, or bathtub and 100 CFM to a jetted tub.
• Multiple fixtures: add them up. A large bathroom with a jetted tub and a separate shower needs meaningfully more airflow than its square footage alone suggests.

Check the CFM number against the square footage the manufacturer rates the fan for. The Broan-NuTone AE80B moves 80 CFM and is rated for rooms up to 75 square feet. A fan advertised for 70 square feet cannot properly serve a 90-square-foot room no matter what the box implies.

Which Fan Fits Your Window and Duct Layout?

A window exhaust fan must match the window opening and the duct route, so measure both before you shop. Confirm the model’s required duct diameter and mounting orientation in its installation manual.

Duct sizing is where most installs go wrong. ENERGY STAR’s certified fan data shows models built around both 4-inch and 6-inch duct connections, and the manufacturer’s spec wins over whatever duct already exists in your wall. Running a 6-inch fan on a 4-inch duct chokes its real-world performance below the rated CFM.

Keep the duct run short, straight, and free of sharp bends. Seal every joint, insulate any section passing through unconditioned space, and finish with an exterior cap that has a backdraft damper.

Vent to the outdoors, full stop. Dumping moist air into an attic, wall cavity, crawlspace, or soffit traps moisture where it feeds mold and rots framing.

If your setup makes a window fan awkward, a wall-mounted model may route more cleanly. The GBR80, GBR80H, GBR100, and GBR100H are wall-or-ceiling options worth comparing. For a shortlist of window-specific units that have been tested in real bathrooms, see our tested bathroom window exhaust fan roundup.

What Do Sones, Controls, and Efficiency Actually Change?

Noise is rated in sones, and lower is quieter. The AE80B lists at 0.7 sones, quiet enough to run during a shower without shouting over it. A fan at 2 sones or higher is noticeably loud in a small room.

Controls decide how often the fan actually runs. Humidity-sensing models turn on automatically when moisture climbs, which fixes the most common failure: people simply not running the fan long enough. The GBR80H, GBR80HLED, GBR100H, and RVSH80 all offer humidity sensing.

Selectable-speed models give you flexibility when your sizing math lands between standard sizes. The Panasonic FV-0511VQ1 switches between 50, 80, and 110 CFM, and the FV-1115VQ1 covers 110, 130, and 150 CFM.

ENERGY STAR certification signals efficient operation, and certified options include the Broan AE80B, Broan BENL8, ReVent RVL80, and several Delta Breez models.

Which Mistakes Sink a Bathroom Fan Install?

Most failures trace back to a handful of avoidable errors, and each one is cheap to prevent before installation day.

• Sizing by floor area alone in a large bathroom with several fixtures.
• Trusting nominal CFM while ignoring a long, bendy, or undersized duct run.
• Exhausting into an attic or cavity instead of through the roof or wall.
• Overlooking electrical limits. The Broan 765H80L carries a 1,300-watt heater, so confirm your circuit capacity before wiring anything.
• Assuming a price or certification applies everywhere. Retailer prices shift by location, and local code amendments and permit rules vary.

One more: confirm the switch wiring and clearances the manual specifies. A fan that fits the window opening but not the electrical box is still the wrong fan.

Work through these in order — measure the room, calculate CFM, confirm duct size and route, then pick noise and controls — and you will buy the right fan once.