As a simple rule of thumb, the airflow from the air compressor must be at least 3/2 times the minimum requirement of the impact wrench. For example, a 1/2″ drive Impact wrench usually has a requirement of about 5 CFM airflow at 90 psi. So, the airflow rating of the air compressor at 90 psi should be 3/2 * 5 = 7.5 CFM.