The ACH50 a measure used to determine how “leaky” a house is using a blower door by depressurizing or pressurizing the house by 50 Pa (Pascals) with respect to the outside.
In order to get the ACH or Air Changes per Hour number you simply multiply the CFM number required to keep it at 50 by 60 and divide that number by the total volume of the conditioned space. For example a 1500 SF ranch house with 8’ ceilings has a volume of 12,000 Cubic Feet of Air. If the CFM reading is 1200 CFM and you multiply that by 60 you get 72,000. Dividing 72,000 into 12,000 would equal 6 ACH.