How do I determine what size of air compressor to buy?

The most important rating for an air compressor is the SCFM output. This is also the input rating that the air tool requires to operate properly. For best results, consider the tool that you anticipate using with the highest SCFM rating and purchase a compressor with an output of 1.5 times that rating. For example, if you plan to use an impact wrench that requires 5.0 SCFM, you would want to purchase a compressor that puts out 7.5 SCFM. If you plan to supply multiple tools simultaneously with compressed air, add the SCFM requirements for all tools and multiply by 1.5.