How to Calculate the CFM of Fans

Measure the length, width and the height of the room with a measuring tape in feet. If measured in meters, covert the dimensions to feet by multiplying by 3.28. Measure any two adjacent walls of the room from one corner to another horizontally. Similarly, measure the vertical height of the room. Do not leave any unmeasured space between the corners.

Multiply all the three dimensions to get the volume of the room in cubic feet. This is the amount of air required to be circulated by the fan. For a room measuring 15 feet in length, 12 feet in width and 12 feet in height, the volume is 2160 (15 x 12 x 12) cubic feet.

Calculate the fan cfm by dividing the volume of the room with a factor called "minutes per air change". Minutes per air change is the time in minutes in which the air of the room should be changed by the fan. Generally the value for this factor lies between 5 to 7 for rooms like living rooms and rest rooms where large amounts of fresh air are needed. The values are comparatively lower for the rooms with less fresh air requirement, like compressor rooms, laundries and boiler rooms. The room with volume 2160 cubic feet requires the fan of cfm 432, (2160/5).

Check the minute per air change factor for your room with the exhaust/intake ventilation guide or seek recommendations from the fan manufacturer. The correct determination of this factor is very important for proper air circulation in the room.