How to calculate the CFM needed for your attic application
One square foot of unobstructed inlet area is required for every 300 CFM output.
Multiply your attic square footage by .7 for minimum air delivery need to exhaust the attic. For darker roofs add 15% to the CFM you just calculated. For exceptionally large attics use 2 fans and have sufficient exhaust or it will put the heat back down into the house.