The Basics: 

CFM stands for cubic feet per minute and is the measurement of airflow in or out of a space. Bath fans are rated according to CFM and typically rated as low as 30 and as high as 180. Of course the ratings get much higher than 180, but for a typical bath fan anything over 180 is usually overkill.

A good starting point to determining the CFM specific to your needs is to calculate 1 CFM to 1 SQ FT. So, for example, if your bathroom is 80 SQ FT 80 CFM that would be a good starting point.

Calculate Size Based on Your Needs

While square footage is a great place to start, there are many other factors to consider when choosing the right size bath fan. For example, if you live alone and have an 80 SQ FT bathroom and live in a typical home or apartment, than 80 CFM should work just fine. On the other hand, if you have a family of 4 or more that likes to take long, hot showers in that same size, 80 SQ FT bathroom, you'll need the extra air movement to remove the excess moisture and prevent mold growth.

 Other Factors to Consider When Choosing a Bath Fan

Location of the home or bathroom- Certain locations, such as a concrete slab foundation laid at the foot of a hill, tend to retain more moisture than others, thus requiring either continuous venting or greater CFM. Further, the proximity of the bathroom to other rooms, particularly the dining room or kitchen, may require a higher CFM to eliminate embarrassing odor creep. 

Static Pressure and Duct Run- The longer the duct run (the length of ducting from the fan to the outside fan vent), the more resistance (static pressure) a fan must overcome. In general, the longer the duct run, the more CFM is needed to compensate. For these types of installations there are usually two options: purchase a higher CFM rated fan or purchase a fan thatself-compensates for this resistance. These fans work by measuring the speed and amperage of the unit and automatically compensate for any loss of flow. 

Features- Bath fans can come with an assorted amount of features such as LED lights and/or night lights, a heater, time delay, humidity sensor, motion sensor and variable speeds.