How much Compressed Air do we ACTUALLY need ?
This is usually measured in Cubic Feet per Minute. a.k.a. CFM at the Compressor itself. For Example: Lets say that we have to run a paint spraying gun which uses about 7 CFM, and a 1/2 inch impact wrench for removing wheel nuts, which uses 8 CFM.
And we will probably be using both at the same time…………this means that I will be needing 15 CFM of Free Air Delivered, (FAD) to run both of these items.

But, and there is a big but here, Air Driven Tools are measured by the amount they require called Free Air Delivered (FAD), while Air Compressors are measured by the amount of AIR DISPLACED.
AND because Piston Type Air compressors only work to 85% efficiency (Sometimes less) you will need an Air Compressor that DELIVERS (FAD) enough air to run the tools.

So lets take a look at this example and work out what size Air compressor we would need:
7 CFM Required for Paint Spraying Gun
8 CFM Required for 1/2 inch impact wrench

15 CFM Total Air Requirement divided by 85% = 17.65 CFM (Round it up to 18 CFM)
So to run these 2 Air Driven Tools at the same time you would need an 18 CFM compressor 

Make sure you know your requirements before pressing the buy button.