Calculate the room volume. Measure the length, width and height of the grow area, then multiply these three numbers to calculate the room volume. For example, if a room is 10 feet long, 8 feet wide and 12 feet high, the room volume is 960 cubic feet (10812).
Calculate the fan size. A cubic feet per minute (CFM) number represents the amount of air a fan moves in one minute, minus any restrictions to the air flow. A general recommendation is if your grow room is in a hotter climate, refresh the air every minute; for a cooler area, refresh up to every three minutes. For example, if your grow room is 960 cubic feet and you're located in a cooler area, select a 320 CFM fan (960/3).
Adjust CFM for any restrictions to air flow. The calculations in Step 2 are for air flow with no restrictions. If a 320 CFM fan is connected, for example, to a carbon filter tube, add 20 percent to the fan size, and if your grow room is in a hot, humid climate, add 40 percent. When selecting a fan CFM, keep in mind it's better to have more circulating air than not enough. For professional suggestions on CFM fan sizes for a grow configuration, consult with a hydroponics store.
Set up the fans. Keep in mind that hot air rises and colder air settles. Therefore, mount one fan higher inside your grow room to blow the heated air out (for example, through a window or hole in the wall to the outside). Mount another fan outside the room to blow cool air into it.