CFM means Cubic Feet per Minute. CFM helps determine what size fan will suit your crawlspace ventilation needs. When purchasing a fan you want to know how much air the fan will move. Air movement is measured in CFM. To calculate the volume of air in the crawlspace you multiple the height x width x length of the crawlspace. If your crawlspace is 40’ long, 20’ wide and 3’ high the volume of air in the crawlspace is 2,400 (40x20x3) cubic feet. If you want to replace all of the air in this crawlspace every 4 minutes you need a 600 (2,400/600) CFM fan.