How to Choose a New Bathroom Fan

What to look for and how to select the perfect bathroom exhaust fan for your needs

Like so many other things plumbing-related, the full significance of the bathroom exhaust fan is too often forgotten, overlooked, or misunderstood. Sure, we love them when an otherwise foul and unusable bathroom is cleared of its funk within a few minutes, but outside of that? In fact, a surprising number of people appear to believe that the fan's primary function is simply to prevent embarrassment and dry heaves. The truth is that that little unit is doing a whole lot more for your bathroom than you might realize!

Humidity is the sworn enemy of your bathroom: its sole objective to deposit moisture anywhere and everywhere, initiating ruin by way of water damage, mold, and mildew. Unfortunately, the "bath" in a bathroom is a traitorous water-vapor machine. Before long, without proper ventilation, paint or wallpaper can start peeling, and moisture can penetrate even to the drywall and wood, leading to costly damage. Windows are the simplest way to evacuate the moist air, but they're inefficient and at times undesirable (like when it's freezing outside). To most effectively dry out the room and avoid wasting heating/cooling energy, a ceiling exhaust fan is needed. Whether you're doing a new install, or replacing a fan turned noise-machine, we're here to help you choose wisely.

Selecting a Bathroom Ventilation Fan

CFM & Air Circulation

How many square feet does your bathroom measure? If you don't know, dig through that junk drawer and grab the tape measure. Bathroom fans have three key ratings to pay attention to, the most important being air flow capacity. This is measured in cubic feet per minute (cfm), and refers to the volume of air circulated. Since that's kind of the point of the fan, it's crucial to find one with a cfm rating appropriate to the size of the room.

The Home Ventilating Institute (HVI) – the certification authority for products like this – suggests bathroom fans should replace the air in a room at least 8 times an hour to ensure proper ventilation. For bathrooms up to 100 sq ft, this works out to about 1 cfm for every square foot: an 80 sq ft bathroom would need at least an 80 cfm rated fan. Those in particularly humid areas may want to increase this ratio to 1.5 cfm/sq ft.

For larger bathrooms, you can obtain the minimum cfm rating using the equation: L x W x H x 0.13. Round the result up to the nearest 10. A single fan with the appropriate rating can be installed, or you can split it up over a few fans; it's entirely up to you. You can also get a cfm rating for bathrooms over 100 sq ft based on the type and number of fixtures in the bathroom:

  • Toilet: 50 cfm
  • Tub/Shower: 50 cfm
  • Jetted Tub: 100 cfm

Simply add up the cfm values for all of your fixtures to obtain a minimum rating.


Although a loud fan may have its virtues (in a quiet house after a particularly volatile meal, let's say), most prefer something quiet. The noise output (as it's perceived by us) of bathroom fans is measured in sones, with 1 sone being something like a quiet refrigerator, and 4 sones a television at normal volume. The sone scale is a linear one, so a fan rated at 3 sones will be three times louder than a 1 sone fan. Noise output doesn't necessarily have any bearing on performance or efficiency, but for many it's the determining factor. Fans that are too loud often fall out of regular use fairly quickly.

Some fans are designed to run continuously. In this case, you'll want at least a 20 cfm rating whatever the size of the room. Because it runs continuously, the fan does not need to be as powerful as one used periodically. Look for a sone rating of 1 or less: if it's on all the time, you definitely want it to be quiet.


The third thing you'll want to pay attention to before selecting a new fan is its efficiency. Most fans made these days don't use very much energy at all, with the majority clocking in under 40 watts. Whatever you end up with, chances are it won't be too much of an energy hog. You'll probably want to be more selective if you're going with a continuous-use model. Fan (or airflow) efficiency is measured in cubic feet of air circulated per minute, per watt (cfm/watt). A higher cfm/watt value means more air is moved for every watt used: a more efficient fan.

Special Features

Now that you know the specifications to pay attention to, it's time to start shopping! There are many additional features available, most of which have no bearing on performance and are a matter of preference. There are exceptions, like timers or off-delays, which ensure the fan runs for an adequate duration (fans should run for at least 20 minutes after a shower). Humidity sensors, which detect the humidity in the room and run or shut off the fan accordingly, should also be considered.


Once you've found the right unit for your bathroom, all that's left is installation. This is a task that's probably best left to professionals, unless you're a seasoned DIYer with electrical skills. Whoever installs it, you'll want to be sure that:

  • the fan is placed in a good location (usually over or near fixtures, or the center of the ceiling, away from openings)
  • controls are effective and convenient (unless using a delay-timer, it's best to have separate switches for lights and fans since it's recommended they run for 20 minutes after a shower)
  • it's vented outside the home (never to an attic or elsewhere)
  • a wall or roof vent cap is also installed to keep your home sealed when the fan isn't in use

With the possible exception of the kitchen, the bathroom is the most important room in a home. Protecting it from the detrimental effects of its often humid environment is paramount, and best achieved with a properly-sized ceiling exhaust fan. You know enough about airflow, sones, and efficiency to find a unit that's perfectly suited to your needs, so buy with confidence and dry that bathroom out!

Ready to shop for a new bathroom exhaust fan?

You may also be interested in these related items:

return to top ↑

Copyright© 1995-2024
All Rights Reserved.