The size of the air compressor required for an impact wrench usually depends on the size of the impact wrench. The average air compressor required is 4.5 CFM @ 90 PSI for a ½ “impact wrench. For example, an impact wrench for a 1” drive requires 10 CFM @ 90 PSI and a ½ “drive requires 10 CFM @ 90 PSI. 4 CFM @ 90 PSI is required. ⅜ requires 3 CFM @ 90 PSI. Furthermore, there are other various determinants such as the type of project, the type of impact wrench, the power supply, and the space. And where you use it.