Vent fans are rated by the number of cubic feet of air per minute (cfm) they can replace or move. You need a fan that will replace the volume (cubic feet) of the growing area in about 5 minutes or less. The air that is pulled out is immediately replaced by fresh air which is drawn from little cracks under the doors or window sills. If your grow room is sealed tightly then an intake fan will probably be needed to bring in fresh air. 

A vent fan is able to pull air out of the grow room more efficiently than a fan is able to push it in.
To calculate the room size multiply width by height this will give you the total cubic footage of your grow room, for example 10' wide by 10' long by 8 tall = 800 cubic feet, Remember that you want your fan to exchange the air within 5 minutes, so for a room that is 800 cubic feet a fan that is capable of moving 160 cfm is need.  If you need some help with this we are here to help so let us know.

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