There are a few things that contribute to the noise level of a bathroom fan. If you want an effective fan the relevant number you're looking for is the CFMs. We previously explained CFMs (in reference to kitchen exhaust fans) as:
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!
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