CFM and sizing

CFM at 90 PSI. Volumetric flow - how much air the compressor delivers per minute at the pressure tools are rated at. This decides which tools run. Always read it at 90 PSI, because a figure quoted at 40 PSI is a larger number describing a less useful condition.

Duty cycle. The proportion of the time the pump is designed to run. Of the sixteen compressors we catalog, three publish anything on this - a 70/30 figure, a 30 to 60 minute continuous limit, and a 25-minute-at-30-PSI rating on a 12V unit. The other thirteen are silent, which means you do not know the limit.

Stored air, as tank gallons times maximum PSI. This decides how long you can draw more air than the pump makes. It is the number that explains why a 6-gallon tank at 200 PSI holds more usable air than an 8-gallon tank at 120, and why comparing tanks by volume alone misleads.

Every air tool falls into one of three duty classes, and the class matters more than the CFM figure. Burst tools draw air for a second or two - impacts, nailers, staplers. Intermittent tools draw for several seconds with gaps - ratchets, air hammers, grease guns. Continuous tools draw for as long as the trigger is down - die grinders, sanders, cut-off tools, spray guns.

For burst tools, tank storage does most of the work and a modest pump is genuinely adequate. For intermittent tools the pump has to be in roughly the right range or the tank drains faster than it refills. For continuous tools the tank is irrelevant after thirty seconds and the pump's output is everything.

This produces one counterintuitive result worth knowing: a 3/8-inch air ratchet at 3 to 4 CFM published is harder on a small compressor than a 1/2-inch impact wrench at 4 to 5 CFM, because you hold a ratchet trigger for ten or twenty seconds rather than one. Size your compressor for the ratchet and the impact takes care of itself.

We size against the highest published demand for the tool class, multiplied by 1.25. The margin covers three real losses that published specs exclude: pressure drop through hose and fittings, a pump delivering less than rated once it is hot, and the slightly thirstier tool you will buy next.

We use 25% rather than the 50% or 100% some industrial guides recommend, and the reason is honest rather than generous. Those larger margins assume continuous shift work with several tools running simultaneously. A one-person home garage uses one tool at a time in bursts, so the tank covers the peaks. Over-margining is exactly the overbuying this site exists to stop.

Every can-run verdict elsewhere on this site is computed from this same dataset with this same margin, so the pages cannot contradict each other. If our CFM chart says a die grinder needs 6 CFM, every compressor page applies 7.5 CFM as the bar - automatically, not by hand.