All Can-Filters® come with a maximum exhaust and maximum recirculation rating, as long as you do not exceed the maximum exhaust rating when matching your fan to your filter you can be confident that the air passing through your filter will exhaust out as clean air. Calculate the cubic feet of the room (Length x Width x Height) and divide that number by the number of minutes you would like to exchange the air in the room (1 – 3 minutes is recommended) that gives you the recommended minimum CFM that you need to exchange the air in that room. (Length x Width x Height) ÷ 3 = Minimum CFM Eg) 10ft x 10ft x 20ft = 2000 Cubic Feet 2000 Cu.ft ÷ 3 = 667CFM A 667 CFM filter is minimum filter size for this size area. Match fan to filter. Consult performance charts to determine which fan will meet the needs of your application.