What CFM air compressor do I need?
Demand from your tools, with duty cycle, and what the tank covers when it runs short.
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
| 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.