SCFM, CFM and displacement CFM
A compressor listing can quote three different numbers, all called some kind of CFM, differing by nearly half. Knowing which one you are looking at is most of buying well.
The three numbers
Displacement CFM
Piston area times stroke times speed. It is what the pump would move if air were incompressible, nothing leaked and nothing got hot. All three are false, so real output is lower: about 70 percent for a single-stage machine and 80 percent for a two-stage one.
It is not a lie, exactly. It is a real measurement of the pump. It is simply not the amount of air that comes out of the hose, and it is the larger number, so it is the one that ends up in listing titles.
SCFM, standard cubic feet per minute
Air delivered, corrected to standard conditions so two machines can be compared. This is the number that matters, and it is always quoted at a pressure. "5.1 SCFM at 90 PSI" is a complete specification. "10 CFM" on its own is not.
ACFM
Volume at actual local conditions rather than standard ones. It matters at altitude and in industrial specification, and rarely in a home workshop.
What the gap looks like
| Delivered SCFM at 90 PSI | Advertised as displacement, single stage | Two stage |
|---|---|---|
| 2.6 | 3.7 | 3.3 |
| 3.5 | 5.0 | 4.4 |
| 4.0 | 5.7 | 5.0 |
| 5.1 | 7.3 | 6.4 |
| 6.4 | 9.1 | 8.0 |
| 7.8 | 11.1 | 9.8 |
| 10.5 | 15.0 | 13.1 |
Pressure changes the answer too
A compressor moves roughly a fixed volume per revolution, so the air it delivers falls as discharge pressure rises, in proportion to absolute pressure. The same machine rated 5.1 SCFM at 90 PSI delivers:
| Pressure | Delivered SCFM | Typical use |
|---|---|---|
| 40 PSI | 9.8 | HVLP spray gun |
| 90 PSI | 5.1 | Most air tools |
| 100 PSI | 4.7 | Framing nailer |
| 125 PSI | 3.8 | Storage pressure |
| 150 PSI | 3.2 | Storage pressure |
This is why a spray gun at 40 PSI is easier on a compressor than its raw CFM figure suggests, and why running tools at 125 PSI when they only need 90 wastes capacity.
How to read a listing
- Look for "SCFM at 90 PSI". If a listing gives that, it is being straight.
- Treat a bare "CFM" with suspicion, especially a large one with no pressure attached. Divide by about 1.43 to guess the delivered figure.
- Ignore horsepower. Peak and "developed" horsepower ratings on consumer compressors are close to meaningless. Air delivered is the specification.