Compressor sizing turns on one question most charts skip: does the tool run in bursts, or continuously? An impact wrench and a sander can draw similar air on paper and need completely different machines.
Check which CFM a listing is quoting. Displacement CFM is the theoretical swept volume of the pump, and a single-stage compressor delivers only about 70 percent of it. A machine advertised at 7.3 CFM displacement is the same machine as one honestly rated 5.1 SCFM at 90 PSI. How to tell them apart.
Two tools, two answers
Why duty cycle decides the compressor| Tool | Nameplate | Duty cycle | Sustained draw | Compressor |
|---|
| Impact wrench, 1/2 in | 4.5 CFM | 20% | 0.9 CFM | 2.6 SCFM |
| Random orbital sander | 6 CFM | 90% | 6.0 CFM | 10.5 SCFM |
The nameplate figures differ by a third. The compressors differ by a factor of several, because the impact wrench is running perhaps a fifth of the time and the sander is running all of it. A continuous tool has no idle time for the tank to refill in, so the compressor has to cover its draw outright.
This is the whole reason people buy a compressor that handles their nail gun beautifully and then find it cannot run a sander for two minutes.
What the tank does, and what it cannot do
The tank is a buffer, not a supply. It covers the gap between what a tool takes and what the pump makes, and then it runs out. A 60 gallon tank cycling between 150 and 100 PSI stores about 27.3 cubic feet of free air, which covers a 5.0 CFM shortfall for 5.5 minutes.
After that you are working at whatever the pump delivers, which for a continuous tool means stopping and waiting. A bigger tank buys a longer first run, not a bigger compressor. Work out your own runtime.
The margin
Conventional practice adds 30 percent or so over calculated demand, for hose losses, fittings, leaks, and the fact that published tool figures are measured optimistically. A compressor running at its absolute limit runs hot and continuously, which is how they die.
Frequently asked questions
What size air compressor do I need to paint a car?
Size it on the spray gun, and size it on the gun's continuous draw rather than its peak. An HVLP gun takes about 9.0 CFM at 40 PSI and sprays for minutes at a time, so with the usual 30 percent margin you need roughly 11.7 SCFM, which is a 14.0 SCFM compressor. That is a 60 gallon two-stage machine, not a pancake. The bigger trap is the preparation: a dual action sander takes 11.0 CFM at a 90 percent duty cycle, so sanding the panels needs about 14.3 SCFM, more than the spraying does. Size for the sander and the gun comes free.
Is a higher CFM always better on an air compressor?
More CFM is never a problem in itself, and it is not free. A compressor that delivers more air costs more, draws more current, is heavier and is louder, and none of that helps once it comfortably exceeds your largest continuous tool. What is worth paying for is honest CFM: a rating at 90 PSI from a manufacturer that publishes delivered SCFM rather than displacement. A 5 SCFM machine rated properly beats a "10 CFM" machine rated on piston displacement, which is roughly the same compressor described flatteringly.
What size air compressor do I need for a framing nailer?
Almost any of them, and the reason is duty cycle rather than CFM. A framing nailer takes about 2.2 CFM while it is firing, but it fires for a fraction of a second, so over a working hour the averaged draw is only 0.1 SCFM even with the margin applied. A 6 gallon pancake keeps up with one nailer all day, and with two on the same hose. What actually matters for a nailer is PRESSURE, not volume: it wants 100 PSI at the tool, so the tank has to cut in before it drops below that, and a long or narrow hose costs you more pressure than the compressor size does. The tools that need a real compressor are the ones with a continuous duty cycle, which is sanders, grinders, blasters and spray guns.
What size air compressor do I need to blow out sprinklers?
Volume, not pressure, and more volume than most homeowner compressors have. Blowing out an irrigation zone wants 20 to 50 CFM at a low pressure, typically 50 PSI and never above 80 for polyethylene pipe. A 5 SCFM garage compressor will eventually clear a small zone by cycling, badly and slowly, and a 6 gallon pancake will not clear one at all. This is the job people rent a tow-behind compressor for, and it is cheaper than buying a machine you use once a year.
What determines the CFM of an air compressor?
Pump displacement, how many stages it has, and the pressure it is delivering against. Displacement is bore, stroke and RPM, which is the theoretical figure. Real delivered air is lower because of clearance volume and blow-by: a single-stage pump delivers around 65 to 75 percent of displacement and a two-stage around 75 to 85. Delivered air also falls as pressure rises, which is why every honest rating names a pressure, and why "CFM" with no PSI attached is not a specification.
The three numbers explained.
What size air compressor do I need for impact wrench use?
Less than most people expect. A 1/2 inch impact wrench draws 4.5 CFM but runs about 20 percent of the time, so sustained demand is around 0.9 CFM. With a margin that is roughly 1.2 CFM, which even a small compressor covers. A decent tank matters more than the pump here.
What size air compressor do I need for a sander?
Considerably more. A random orbital sander draws 6 CFM continuously, so the compressor must supply all of it rather than an average. With a margin that is about 7.8 CFM, which means a 10.5 SCFM machine or larger. Sanders and grinders are what size a workshop compressor.
Does a bigger tank mean I need less CFM?
No. A bigger tank buys a longer first run before the compressor has to keep up, which helps enormously with intermittent tools and not at all with continuous ones. Once the tank is down to cut-in pressure you are working at whatever the pump delivers.
How much CFM do I need for air tools generally?
Add the sustained draw of the tools you run at once, then add a margin of about 30 percent. For intermittent tools the sustained draw is nameplate times duty cycle; for continuous tools it is the full nameplate figure.