How many CFM

Im just wondering home many CFMs i need for a grow box that is 4 feet x 5 feet x 3 feet with a 400 watt hps in it?
Also about how many CFMs are 90mm 12v computer fans?
Thanks guys

CFM on computer fans varies, here is a guide that is pretty useful

It really depends on how you use it.

Do you really need to remove heat from the whole room (assumes that is what you are trying to accomplish). Why no isolate the heat source in a sealed hood and draw air across your bulb FROM OUTSIDE YOUR ROOM and vent it TO OUTSIDE YOUR ROOM. Then use a smaller fan to move fresh air in and odors out. this is where you put your carbon filter.

Hope I did not make too many assumptions

Im acctually more conserend about humidity. cause i have a box for veg that is 2.5 feet x 4 feeet x 3 feet and it has a 90mm computer fan for exhause and its humidity is about 75 to 80ish whish is going to be to high for flowering.

Whats the humidity where the Air is coming in at?