From the link: Growing Environment Volume: Calculate the volume of your room by multiplying the length x width x height. For example, if you have a grow tent that is 8’ x 8’ x 7’, then the volume would be (8x8x7) = 448 cu./ft.
Volume To CFM: Your exhaust fan should be able to exhaust the equivalent of your growing environment’s volume in two minutes, or one minute if you are having issues with high temperatures. Assuming we are not having issues with heat, the growing environment volume from our example is 448 cu./ft., which means we need to exhaust 448 cu.ft. / 2 minutes = 224 CFM. From this I know that I need a 224 CFM fan at a minimum.
Other Variables To Account For: We’re not done yet. Now to account for other variables that can require you to have more CFM required for your grow room:
Lights: For each air-cooled light (600-1000w) add the equivalent of 10% of the calculation you finished in the ‘Volume To CFM’ step. For non air-cooled light add 20% per light.
CO2: Add 10% for rooms with CO2 enrichment via a CO2 burner or generator (they create heat).
Carbon Filters: Add 20% if you’re using a carbon filter.
Ambient Temperature: For hot climates (Southern California, Arizona, etc…) add 25%. For humid and hot climates (Florida, Georgia, etc…) add 40%.