Check-out the info in the GP FAQ
What size vent fan do I need
Here’s how you can calculate the amount of ventilation you need. If for instance you wanted to keep you grow room temperature from getting any more than 5° warmer than the intake air temperature, and you were using 400 watts of power, you’d make the calculation below in blue. The chart is based on the following formula. It is a well-established heat transfer formula.
(3.2 × 400) ÷ 5 = 256 