Best Air Compressor for Impact Wrench (Size & CFM) - Fine 2019/09/28· 27 gallons Compressor: Click to See details. In my opinion, the minimum air compressor for impact wrench is 6-gallon one with 4 CFM. Any lower than that, you will really struggle to use the tool. Remeer that this is the minimum size. I strongly recommend you buy one with a tank capacity of 20-gallon or more.