i wouldn't think any less than 150 cfms but there's a lot of factors to consider, such as where the incoming air will be coming from and where the outgoing air is going.it may take even more ,i'm just not sure. i have a 4x3 grow cabinet and i put a 50 cfm bathroom fan in it venting to the outside of my outdoor shed.it's been more than adequate so far and i put it on a rheostat (dimmer) so i could fine tune it for the ambient temp and sound.I will be upgrading in the next week or two to a 400 watt HPS myself and i may very well find that a 50 cfm exhaust is not nearly enough. my only saving grace if there is one is that my grow is not inside my home and in an outdoor storage shed which is uninsulated.that makes it even trickier but we'll see.the more control you have over the environment ,the better your crop will likely be. hope this helps.happy growing!!...Dee