If your kitchen is 12' x 15' and 10' high, that equals 1,800 cubic feet. To find the size of fan needed, multiply the cubic feet x the # of air exchanges (15) then divide by 60 (minutes in an hour). In our example that would be: 1,800 x 15 = 27,000 divided by 60 = 450. You need 450 CFM minimum for this size of kitchen.