I've got a 4 Gallon, and two 21 Gallons. The 21 Gallons run fairly constantly when Im on the grinders and sanders. They are 9cfm at 90 PSI. The 21 Gallon works, but after awhile it gets annoying. I'd prefer a 60-80 Gallon. The 21 Gallons are like $350, the 60-80s are like $400-$500. So I would have spent the extra cash if I wasnt given a 21, and if I had the space.