How do i find how many cfm's i will need?

i am going to be using a 400w GPMH and HPS for my second grow. i have a closet that is approx. 2' deep x 4' wide x 8' tall. How many cfm's do i need to keep this area at a good 75F???

http://boards.cannabis.com/indoor-gr...ml#post1634834

that is wonderful, very informative link. Do u have any advice on what to use to keep my intakes light proof, as i didnt see anything on ur link about that?

Anything?

Depends a lot on what the intake looks like- if yer using duct already, a 90 degree bend facing down will do the trick in most cases. Remember to allow for drag when figuring intake size.

With more square openings, a flap of panda or other light-proof plastic over the intake ( oversize by a couple inches, tape to wall only over vent area, wieght bottom flap slightly) ( You make it long to allow ends to distort and absorb flex) works well in many cases.

You might also want to google up a diagram of a "darkroom blackout vent" . ( Ya didn't hear it from me, but this design works jut fine when made out of a card-board box.

Again, allow for drag from blackouts- you should prob toss on another 10-20% as a load factor in the fan-sizing and intake sizing math.

Also, should point out that cfm does'nt translate into room temperature unless you know the intake temperature - there's that whole differential temperature thing, which gets into a fair bit o' math.
(Also, I don't know how to make the little triangle delta symbol on this keyboard, which makes the equations pretty unreadable.)

But obviously, you'll need fewer cfm if your intake air is 50F than if your intake air is 74F, to remove the same # of calories from the room.

Yea i am kind of space challenged so i dont think i would have room outside of my area to have duct bent around. If i put a vent on the bottom of the room and made sure the vent fins angled up so that light would not enter from my bedroom lights, do u think that would do the tirck?