CFMs, or Cubic Feet per Minute, is a measure of volumetric flow. In the world of fans it's used to tell how powerful a fan is. The higher the CFM, the more air that fan pushes or pulls through. Depending on the application, there are specific requirements for how effective a fan must be.
Home Depot explains that for bathrooms under 50 square feet a 50 CFM fan is more than adequate. For anything over 50 square feet they recommend approximately 1 CFM to 1 square foot ratio.
The problem with a high power fan is usually that they generate a lot of noise, measured in sones. 1 or so sones is a fairly quiet fan, although everyone's opinion of "quiet" is different.
For a little bit more money you can get an extremely quiet fan such as these models at only .3 sones that still pump 80 to 110 CFM!Broan QTXE080 Ultra Silent Bath Fan $105.18 This white grille Broan model vents 80 CFM yet is rated at only .3 sones of loudness. This model is rated for use over a tub/shower as well with a GFCI circuit. Panasonic FV-11VQ5 WhisperCeiling Fan $111.07 Step up to 110 CFM while maintaining that low .3 sone rating. This Panasonic uses a double hanger bar system that allows you to position the fan anywhere in your ceiling between two joists instead of having to nail it directly to the side of a joist. It's also rated for tub/shower use.
Don't forget to check your duct size to ensure compatibility and space as well as whether you need lighting and if the electrical is properly in place.
MORE FANS ON APARTMENT THERAPY
• Not All Exhaust Fans Are Created Equal: It's All About the CFMs
• Ventilation Fans in the Bathroom
• Not Too Ugly Bathroom Fans