This has come up a lot lately so here it is Heres an easy calculator for your compressor fill time from empty http://www.egnergy.com/calculator.html To calculate fill time while spraying just subtract .5cfm from the compressors input cfm for an airbrush, 1.1cfm for a fan brush or the listed cfm for a spray gun. And remember to set start pressure at when it kicks on, not zero. Use your compressors cfm rating at 40psi if you have it Bear in mind, many european compressors and chinese jobs are sometimes rated by the air they take in, this is a no load rating that means very little in the real world. Its output at 40psi can be 2/3rds or even half of that. If you arent sure, simply compare to brand name american compressor with the same motor power and if the compressor your looking at seems way higher cfm then chances are it's rated this way (use the numbers from the american job for an estimate). For drain times ((PSI x tank volume in Cf) / 14.7) / cfm consumed or ((PSI x tank volume in L) /14.7) / lpm consumed This is to drain the entire tank. To determine if the compressor is adequate (assuming the normal 50% duty cycle) Use the difference in pressure between full and kick on in the above formula in the place of PSI If fill time while spraying is equal to or greater than your result you are good to go. If its a little less you're OK (assuming your not constantly spraying) If its half or below...its gonna struggle... but could be ok if the drain time is all the longer you intend to spray for or if you do a lot of broken up sprays for detail work at 20psi and less. If its a fourth, forget it , you will definitely hate your compressor. These will give you rough estimates, figuring the real world numbers down to the second would be way too complicated for this purpose. Hope this helps some people.