how many CFM?

sorry for posting this twice but i posted it in grow room design and then relized there werent that many people viewing that section.so my question is I am building a new grow box tomorrow since my wife wants the closet back. it's going to be 2.5' wide, 2' deep, and 6' tall. my last few plants have already bean veging for a while now and will be switched to flower as soon as the box is done. i'll be flowering with a 400w HPS. and was wondering how many cfm my exhaust fan should be? thanks for any advice.

i would say 300 an up is your light air cool then use some inline fans

general rule of thumb for ideal ventilation is exhausting the cabinet 3 times every minute.

Your area is 30 cubic feet (2.5 x 2 x 6) so in order to exhaust that area, you'd want a fan to be rated at least 90 ( 3 times 30 cubic feet ) CFM.

That is assuming your 400w HID is enclosed in glass and is using a separate fan (a 90+CFM fan should work fine here too) to ventilate and cool the light alone.