So you got your grow room setup, lights, pots, and of course, POT! But your missing one thing, and thats ventilation. I'm going to help you along the way to making sure you have the right fans for your situation. Most growers would suggest you replace the air in your system 20 times in an hour, every 3 minutes. Now you are probably asking, how do I do that? Easy, calculate the area of your grow space, we'll use a 2 meter x 2 meter x 2.5 meter grow room for an example. You want that 10 square meters of area to change 20 times in an hour so you will multiple by 20. 10m3 x 20 = 200m3, but "overkill the hell of your ventalation" as suggested by Rumpleforskin so lets just say you want 225m3. Now comes the calculator, to find the correct CFM for the system you want to use take your 225m3 and multiple it by 0.588 225m3 x 0.588 = 132.3 CFM so lets round up to 135 CFM to continue to "overkill the hell of your ventalation" For your 10 square meter grow room, you want a 135 CFM fan to run constantly. And there you have it, now you know what size fans to get for your growroom.