6" inch what I like about it is I can take off the glass have air blowing on the light to kool it but then have another fan using the other end for exhaust for the room. I am buying 2 6" boost fan 250cfm a piece. Then I might use my 4" that i have for my cab now as an intake so we will see what happens I know I am shutting everything down when lights are off except my air pump. Sorry for the hijack SOME.