CFM Calculator

Bathroom exhaust fans don’t require nearly as many CFM as a kitchen range hood, but their importance should not be underestimated. They’re most commonly found in bathrooms and rid them of moist air produced by your bath, shower or sink. If moist air is trapped in your bathroom, it can potentially lead to mold, which is both unsavory and unhealthy.

As noted above the HVI recommends an ACH value of 8 for the bathroom. Bathrooms vary in size, so to ascertain how many CFM your exhaust fan needs to have, try this simple method:

First, determine the size of your bathroom. Measure the room’s dimensions (length, width, height) and calculate its air volume in cubic feet. For those who inexplicably forgot how to find the volume of a cube, simply multiply the three numbers. For example, if your bathroom’s dimensions are 7 x 9 x 8, your bathroom’s air volume is 504 cubic feet. This means the exhaust fan needs to ventilate 504 cubic feet of air in order to achieve 1 ACH.

Remembering that the recommended ACH for bathrooms is 8, the exhaust fan needs to ventilate 504 cubic feet x 8, which equals 4,032 cubic feet, in one hour. Then, exhaust fan ratings are determined by the minute. So, divide 4,032 by 60 (for minutes in an hour) for the desired CFM rating for the exhaust fan, which in this case is rounded up to 68.

Another method assumes you have eight-foot ceilings. In this case, multiply your bathroom’s length by its width to determine its square footage, and multiply it by 1.1. Working off the above example that your bathroom is 7 feet long and 9 feet wide, it is 63 sq. feet. Take that 63 sq. ft. and multiply it by 1.1 to get 69.3. As a result, you’ll need an exhaust fan with 70 CFM to properly ventilate your bathroom. Ceilings which exceed eight feet in height will require additional CFM.

But, if your bathroom is 100 sq. ft. or more in size, the CFM equation is different. Instead of depending on square footage, you’ll have to chime in with the different fixtures in your bathroom to determine the exhaust fan’s proper CFM rating. HVI recommends the following CFM for different bathroom fixtures:

·         Shower – 50 CFM
·         Toilet – 50 CFM
·         Bathtub – 50 CFM
·         Whirlpool tub – 100 CFM

So, if your bathroom is larger than 100 sq. ft. and has a shower and a toilet, you’ll need an exhaust fan with at least a 100 CFM rating. Have a bathtub, as well? The CFM rating gets bumped up to 150. You get the idea.

Start here to get started shopping for the right bath for your home!