According to Northern Tool, for a 1/4" ratchet, like the Ingersoll Rand 105-D2 1/4-Inch Air Ratchet, you'll need between 2.5 - 3.5 SCFM. If your ratchet is 3/8", you'll need a larger size, somewhere between 4.5 - 5 SCFM. Northern Tool also provides a helpful explanation about the SCFM (standard cubic feet per minute) measurement: When matching a pneumatic tool's requirements to an air compressor's capabilities, the most important rating to consider is how much air the compressor can deliver, which is measured in standard cubic feet per minute (SCFM). Tools powered by air compressors also have SCFM ratings, which indicate the air flow required for optimal operation.