Duty cycle is the whole subject
Air tool ratings are quoted as CFM at 90 psi, and almost everyone reads that as the demand the tool places on the compressor. It is not. It is the consumption while the trigger is down, and the fraction of the working minute for which that is true varies from a few percent to nearly all of it depending on the tool.
A framing nailer might be rated at 4 CFM and fire twenty times a minute, each shot lasting a fraction of a second. Its real average demand is a fraction of a CFM, which is why a small pancake compressor runs one indefinitely. An air sander rated at 11 CFM has the trigger down for most of the minute, so its real demand is close to 10 CFM continuous. The pancake compressor delivering perhaps 3 CFM does not stand a chance, and the fact that it managed the nailer tells you nothing about it.
That is the calculation on this page: sum each tool's rating multiplied by its duty cycle, then multiply by a safety factor you choose. Everything else is commentary on that one line.
Rough duty cycles by tool
| Tool | Typical duty | Behaviour |
|---|---|---|
| Brad or finish nailer | 1-3% | Trivial average demand |
| Framing nailer | 3-8% | Bursts, recovers between |
| Impact wrench | 10-25% | Short high draws |
| Ratchet, drill | 25-50% | Sustained but interrupted |
| Spray gun | 50-75% | Continuous while the pass runs |
| Orbital sander, die grinder, cut-off tool | 80-100% | Effectively continuous |
These are working figures, not specifications. Your own habits move them a long way, and the honest use of the table is to pick a starting number and then adjust it once you have watched yourself work for ten minutes.
What the tank does and does not do
A tank stores compressed air, and the amount it stores is small when you convert it back to free air. Sixty gallons is 8.02 cubic feet of tank. Between a 115 psi cut-in and a 145 psi cut-out it holds about 16.4 cubic feet of free air, because the pressure difference of 30 psi is roughly two atmospheres. At a 5 CFM pump that is about three and a quarter minutes to refill.
Now run a 10 CFM sander from it against that same 5 CFM pump. The deficit is 5 CFM. Starting at 145 psi and letting it fall to 90, the tank yields about 30 cubic feet of free air, which lasts about six minutes. After that you are running at whatever the pump delivers and the tool is working at reduced pressure, which is to say badly.
The tank bought six minutes. It did not buy any capacity. This is the single most useful thing to understand when comparing compressors, because tank size is the number most prominently advertised and delivered CFM at 90 psi is the number that decides whether the machine can do the job.
The figures that get quoted at you
Horsepower on a compressor is close to meaningless as a comparison because the way it is measured varies enormously. What matters is delivered air at a stated pressure, usually written as CFM at 90 psi or SCFM at 90 psi. If a specification gives you only horsepower and a tank size, it is not telling you what you need.
Then subtract reality. Hose diameter and length cost pressure, quick-connect fittings cost more than people assume, and every joint in a shop air line leaks a little. That is what the safety factor covers, and 1.25 is a modest allowance while 1.5 is realistic for an older system with a long run to the far end of the shop. If the compressor also feeds a shop air cleaner or a spray booth, look at the air changes calculator for the ventilation side and the dust collection calculator for the chip extraction side, because neither of those runs on compressed air and both compete for the same electrical supply.
Questions people ask
What size air compressor do I need for an air sander?
Look up the sander CFM at 90 psi and assume a duty cycle of 80 to 100 percent, because a sander is a continuous-draw tool. A common 6 inch orbital rated at 11 CFM therefore needs somewhere around 9 to 11 CFM delivered, and with a modest safety factor that is a machine delivering 12 to 14 CFM at 90 psi. That is a substantial compressor, well beyond a portable, and it is the tool that most often reveals that a shop compressor bought for nailing is not what people thought it was. No tank size fixes this; the pump has to be able to make the air as fast as the tool uses it.
Does a bigger tank mean I need less CFM?
No, and this is the most common misunderstanding in compressor buying. A tank stores a fixed quantity of air determined by its volume and the pressure difference it works across, and once that is spent the machine can only deliver what the pump makes. A bigger tank lengthens the burst before you have to wait and reduces how often the motor cycles, both of which are worth having. What it cannot do is raise the average delivery. If your average demand exceeds delivered CFM, a larger tank changes how long you work before the problem appears, not whether it appears.
Why is horsepower not the number to compare?
Because compressor horsepower is measured inconsistently and has been used as a marketing figure for a long time. Peak or developed horsepower on a consumer machine can be several times what the motor draws continuously, and two compressors advertising the same horsepower can deliver very different amounts of air. Delivered CFM at a stated pressure is the specification that means something, because it is measured output rather than input. If a listing gives horsepower and tank size but no CFM at 90 psi, treat the omission as informative.
How do I work out my duty cycle if I do not know it?
Time yourself. Work for five minutes as you normally would and note roughly how much of that time the trigger was actually down. Most people are surprised in one direction or the other: nailing is far more intermittent than it feels, and sanding is far more continuous. If you cannot measure it, use the table on this page as a start and then run the calculation twice, once at the low end and once at the high end of the range. If the answer barely changes, the uncertainty does not matter for your decision. If it changes the compressor you would buy, that is worth five minutes with a stopwatch.
Can I run two air tools at once from one compressor?
The arithmetic on this page will tell you, but the answer usually depends on whether both are continuous tools. Two nailers are almost free because their duty cycles are tiny and rarely coincide. A sander and a spray gun together are two continuous demands added directly, and very few shop compressors handle that. Enter both tools with their real duty cycles and compare the peak draw line against your delivered CFM. If the peak exceeds delivery, look at the burst figure to see how many minutes the tank covers before pressure falls to where the tools stop performing.