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As the trend to larger spa-like bathrooms continues to gain in popularity, the need
for proper ventilation becomes more important.  While many people are tired of their noisy and ineffective bath fan – not many people know how to properly size a fan to their needs.

There are several different ways to calculate the Cubic Feet Per Minute (CFM) of air movement needed to properly ventilate a bathroom. Please refer to the last paragraph of this article for more information regarding static pressure.  For example, a long duct run from the fan to the outside can skew your calculations.

METHOD 1
This method works on the assumption that the goal for bathroom ventilation is 8 complete air changes per hour.  Take your bathroom square footage X ceiling height to get the total cubic feet to be ventilated.

For example:  The bathroom measures 10 feet wide and 12 feet long.  It has 8 foot
ceilings.  So 10 x 12 x 8 = 960 cubic feet. Next, take the cubic feet and divide by 60 minutes.  Then take the result and multiply by 8 (remember, our target is 8
complete air changes/hour).  The complete equation is as follows:

STEP 1: 10 ft X 12 ft X 8 ft = 960
STEP 2: 960 cubic feet, divided by 60 minutes = 16
STEP 3: 16 x 8 = 128 CFM

Therefore we need 128 CFM of air movement to properly ventilate the bathroom in our example.  Assuming there is not and excessively long or twisting duct run to the outside, we would select a fan that moves somewhere around 130 CFM or higher.

METHOD 2
This method is a simple one – for bathrooms under 100 square feet.  According to the guidelines of HVI (Home Ventilating Institute) baths 100 square feet or smaller require one CFM per square foot of bathroom – with a minimum of 50 CFM.

Therefore, if for example you have a 7 ft. X 9 ft. bath – you need 63 CFM.  Most conventional ceiling insert fans however, are rated either 50 or 80 CFM – so you will have to round up or down (rounding up preferred).

METHOD 3
This method of calculation is designed for bathrooms over 100 square feet factoring in the number of fixtures in the bath (toilets, showers, tubs, etc.).  The general guidelines in this method are relatively straightforward:
* Allow 50 CFM for each standard toilet, shower or tub
* Whirlpool and jetted tubs require 100 CFM

Therefore, if you have 1 toilet (50 CFM), 1 shower (50 CFM) and 1 whirlpool tub (100 CFM), you will need a 200 CFM Fan.  An in-line fan kit would also work well in this bathroom. The kit typically includes a 200 CFM fan and allows for 2 exhaust grilles in the ceiling.

Static Pressure and Duct Run:
A ventilating fan must overcome resistance when pushing air from the inlet, through the duct, to the outside of the building. This resistance is known as static pressure. The amount of static pressure depends on the duct length, type of duct, duct elbows and the type of roof jack or wall cap.  Essentially, the more elbows, turns, duct length, etc. you have, the more static pressure will be present and less effective CFM of air movement you will get out of your fan.

There are some user friendly air duct calculators, and other tools available to precisely determine the effective CFM of air movement best suited for your specific situation. Whether you need a fan for your home or commercial building, it is advisable to simply round up to a larger fan, if you know you don’t have a straight duct run to the outside.