How to Calculate Bathroom Fan CFM

An indoor bathroom without a window simply becomes a breeding ground for mold if there is no bathroom fan to take care of the moisture that is accumulated in the air by the shower. An exhaust fan pushes out the moistened air and brings in the fresh one in right amounts at the right intervals of time. Well, that is possible only if the fan is powerful enough to make that happen. For an exhaust fan to be considered functional on the required scale, it needs to replace the air in the bathroom 8 times in the timespan of 60 minutes. In technical terms, the effectiveness of a bathroom fan is measured in terms of cubic feet per minute (CFM) of air moved. Therefore, you need to know the CFM measure of the bathroom fan required at your place so that can you buy the one you actually need. Here is a detailed process that will teach you how to measure the CFM of the exhaust fan required for your bathroom.

1. Figure out the volume of your bathroom

Measure the length, width and the height of the bathroom. Use a measuring tape to figure out the exact dimensions of the walls and the floor. However, if you have a bathroom that isn’t cuboidal in shape, it is advisable to keep a sheet of paper handy so that you can draw out the floor plan to label the walls and make the necessary measurements.

2. Calculate the volume of your bathroom

Well, in simpler words, you have to measure the quantity of solid, liquid, or gas that your bathroom accommodate if it was to be completely filled with the substance in consideration. For example, you can use the mathematical formula length*breadth*height to measure the volume of a cuboidal bathroom. The same formula would apply for a square bathroom as well. However, for differently shaped bathrooms, the floor plan you laid out before will help you calculate the volume as well.

3. Calculate the CFM with the appropriate formula

The volume of the bathroom calculated in step finds its significance in the formula for the calculation of the CFM of the required fan. By simply divide the measures volume by 7.5, you will find out what the CFM measure of the required fan needs to be. For instance, a room with a volume of 750 cubic meters will require a fan with a CFM value of 750/7.5=100.

What is so magical about the number 7.5?

Well, as stated above, for a bathroom fan to function at an optimum level, it needs to completely replace the air in the bathroom 8 times in an hour. In other words, it needs to replace air measuring equivalent to the volume of the bathroom every 7.5 minutes in an hour. It is a standard measure adopted by the industry.

Knowing the CFM is essential before buying the appropriate fan for your bathroom. After all, you would not want to spend time in a bathroom where the air still feels moist even after the installation of a bathroom fan!


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