Bathroom vent CFM calculator uses 3 stage formula located below. It is based on HVI (Home Ventilating Institute) guidelines that recommend approximately 8 room air exchanges per hour for a bathroom under 100 square feet of area.
Any result smaller than 50 CFM would still require minimum 50CFM exhaust fan (building code requirement). HVI ventilation rate recommendation for bathrooms larger than 100 square feet is slightly different and based on the number and type of plumbing fixtures within the bathroom area.
Each fixture (toilet, shower stall , bath tub) requires 50 CFM, Jacuzzi / hydro spa / whirlpool – 100 CFM. All you need to do is add number of fixtures and multiply by the required CFM.
Example of a calculation for bathrooms with over 100 square feet of area:
Toilet (50 CFM) + shower stall (50 CFM) + Whirlpool (100 CFM) = 200CFM bathroom exhaust fan.
Building code does not specify bathroom vent locations but in any size bathroom it is the most logical to install it over or as close as possible to the plumbing fixture that requires it. You can also install two or more individually controlled bathroom exhaust fans over different areas (with combined CFM’s equal to your calculation).
This would be the most efficient way to control moisture and bathroom odors (enclosed toilet rooms require separate exhaust fans).
Based on the final results of the bathroom vent calculator or bathroom exhaust manual calculations – pick the bathroom exhaust fan and install it in the most appropriate location.
This article was written by Dariusz Rudnicki
I'm a retired Illinois home inspector, founder and editor of checkthishouse.com, a blog which attracts around 2 thousand readers daily and is dedicated to answering the many questions of home owners and home buyers. Connect with me on Google+ Find me on Google+ Local