Two compressors, same horsepower, sitting side by side on a spec sheet. One says 17 CFM at 90 psi. The other says 17 SCFM at 90 psi. A customer called us last month convinced they were identical and wanted to know why one cost more. They are not identical, and the reason is one of the most misunderstood things in this business.
Here is SCFM vs CFM in plain terms, why the S matters, what ACFM and ICFM are doing on the same page, and how to use all of it to size a compressor that actually runs your tools.
CFM is a volume. SCFM is an amount of air.
Cubic feet per minute is just volume moving past a point. The problem is that air is compressible, so a cubic foot of it can hold wildly different amounts of actual air depending on pressure and temperature. A cubic foot of air at the top of a mountain in July has fewer air molecules in it than a cubic foot at sea level in January. Same volume, less air.
SCFM fixes that by defining what a cubic foot means. Standard cubic feet per minute is the flow corrected to a set of reference conditions, so the number describes how much air (how many molecules, really) is moving, not how much space it takes up. That makes it the only fair way to compare two compressors, because it strips out the weather.
The catch is that there is more than one set of standard conditions in use, and manufacturers do not always tell you which one they picked.
| Standard | Pressure | Temperature | Humidity | Who uses it |
|---|---|---|---|---|
| CAGI (Compressed Air and Gas Institute) | 14.5 psia | 68 F | 0% RH | Most US compressor manufacturers on CAGI data sheets |
| ASME | 14.7 psia | 68 F | 0% RH | Engineering references, some tool ratings |
| ISO 1217 (metric "free air delivery") | 1 bar (14.5 psia) | 20 C (68 F) | 0% RH | European and Asian manufacturers |
The 14.5 versus 14.7 difference is small, about 1.4 percent, but it is real and it is why two honest ratings can disagree slightly. When a spec sheet just says CFM with no S, it is usually a rounded SCFM, a marketing number, or a displacement figure. Which brings us to the number that fools the most people.
Displacement CFM: the number that isn't delivered
Small piston compressors often advertise a big CFM that turns out to be pump displacement: bore times stroke times rpm, or how much air the cylinder would move if it were 100 percent efficient. Nothing is 100 percent efficient. Real delivered air on a single-stage piston unit is often 60 to 70 percent of displacement once you account for valve losses, blow-by, heat and clearance volume.
So a compressor advertising 10 CFM displacement might deliver 6.5 SCFM at 90 psi. That is a 35 percent gap between what the box said and what comes out the hose, and it explains a lot of angry air sander owners. Always look for the phrase "delivered" or "free air delivery" and a pressure attached to it, like "5.7 SCFM at 90 psi." If there is no pressure attached, the number is not useful.
ACFM: what your compressor actually inhales
Actual cubic feet per minute is flow at real conditions: your altitude, your compressor room temperature, your humidity, today. A compressor is a fixed-displacement machine. It moves a set volume of intake air every revolution, no matter how much air is in that volume. On a hot day at altitude, each cubic foot it inhales contains less air, so it delivers fewer standard cubic feet of compressed air even though it is working just as hard.
Here is the relationship in usable form:
SCFM = ACFM x (P actual / P standard) x (T standard / T actual)
with pressures in psia and temperatures in absolute degrees (Rankine, which is F plus 460). Walk through Denver. Atmospheric pressure there is about 12.1 psia instead of 14.7. On a 95 F day the intake air is at 555 R versus 528 R standard. A compressor that moves 100 ACFM through the inlet is delivering:
100 x (12.1 / 14.7) x (528 / 555) = about 78 SCFM
Same compressor, same motor, 22 percent less air than the sea-level rating on a mild day. If you bought that machine off its 100 SCFM data sheet and sized your tools to 95 SCFM, you are short in Denver every summer afternoon.
ICFM: the one rotary screw buyers should know
Inlet cubic feet per minute is the flow at the compressor's inlet flange, after the intake filter and any inlet valve. It is slightly different from ACFM because the intake filter and piping drop the pressure a bit before air reaches the airend. A dirty intake filter increases that drop and quietly lowers your output. On a rotary screw, ICFM is what the airend is actually sized on, and it is why replacing the intake filter on schedule has a direct effect on delivered air. Our intake filter guide covers the schedule.
Why the pressure attached to the rating matters
A compressor's SCFM changes with discharge pressure. The same piston pump delivers more air at 40 psi than at 90 psi because it is doing less work per cubic foot. That is why reputable manufacturers rate at two points, usually 40 psi and 90 psi. A tool needs its air at a pressure, so the only rating that matters for sizing is the one at or above your tool's operating pressure.
Rotary screws are rated at their full-load discharge pressure, commonly 100, 125, 150 or 175 psi. A 25 hp screw might deliver 105 SCFM at 125 psi and only 95 SCFM at 150 psi. Buy the pressure you need and no more, because every 2 psi of extra discharge pressure costs roughly 1 percent in energy.
How to size a compressor without fooling yourself
Step one is adding up your tools. Use the tool manufacturer's SCFM at 90 psi, which is the industry standard rating point for air tools. Our Air Tool CFM Chart has the common ones. Note that tool ratings are usually average consumption at a duty cycle, not continuous. An impact wrench listed at 5 SCFM might pull 20 SCFM while the trigger is down and zero the rest of the time.
Step two is a use factor. In most shops not every tool runs at once. Multiply your total by something between 0.5 and 0.8 depending on how busy the shop is. A single-user shop can go lower. A production line where every station runs continuously should use 1.0.
Step three is correcting for your conditions. If you are above 3,000 feet or your compressor room runs hot, apply the ACFM correction above and buy a compressor whose rated SCFM covers your need after the correction, not before. A safe shortcut is to add 3 to 4 percent of capacity per 1,000 feet of elevation.
Step four is margin. Add 20 to 30 percent for leaks, growth and filter pressure drop. Then pick the compressor from its delivered SCFM at your required pressure, not its horsepower and not its displacement.
A quick example
Body shop with two techs. DA sander 13 SCFM, HVLP gun 12 SCFM, impact wrench 5 SCFM, blow gun 4 SCFM. Total 34 SCFM. Use factor 0.7 gives about 24 SCFM. Shop is at 5,000 feet, so add 18 percent: 28 SCFM. Add 25 percent margin: 35 SCFM at 90 psi delivered. That is a 7.5 hp two-stage piston or a 7.5 to 10 hp rotary screw. The customer who walked in wanting a 5 hp unit rated "18 CFM" on the box would have been running out of air every time both techs picked up a tool.
Our air compressors collection lists delivered CFM at pressure on every unit, and our piston compressors and rotary screws are broken out separately so you can compare like with like.
Cheat sheet
| Term | What it measures | When to use it |
|---|---|---|
| CFM | Volume flow, conditions unspecified | Treat with suspicion unless a pressure and "delivered" are attached |
| SCFM | Flow corrected to standard conditions (14.5 or 14.7 psia, 68 F, dry) | Comparing compressors and matching tools |
| ACFM | Flow at your actual altitude, temperature and humidity | Figuring real output where the compressor lives |
| ICFM | Flow at the compressor inlet after the filter | Rotary screw airend sizing and intake filter effects |
| Displacement CFM | Theoretical pump swept volume | Never for sizing |
| FAD | Free air delivery, the ISO term for delivered flow at standard conditions | Same job as SCFM on European data sheets |
Frequently Asked Questions
Is SCFM higher or lower than CFM?
It depends on your conditions. At sea level on a 68 F day they are nearly the same. At altitude or in a hot compressor room, the actual volume flow (ACFM) is higher than the SCFM because each cubic foot holds less air. If a spec sheet says plain CFM, it is usually either a rounded SCFM or a displacement figure, and displacement is always higher than delivered SCFM.
How do I convert CFM to SCFM?
Use SCFM = ACFM x (P actual / 14.7) x (528 / T actual), with pressure in psia and temperature in Rankine (F plus 460). For a quick estimate, subtract 3 to 4 percent per 1,000 feet of elevation and about 1 percent for every 5 F the intake air is above 68 F.
Why does my compressor put out less air than it is rated for?
Most often because the rating is displacement, not delivered air, or because it was rated at 40 psi and you are running at 90. Altitude, a hot compressor room, a clogged intake filter, and worn valves or rings all cut delivered air too.
Which rating should I use to size a compressor for my air tools?
Delivered SCFM at 90 psi, because that is how air tools are rated. Add up your tools, apply a use factor, correct for altitude and temperature, add margin, and buy on that number.
Do rotary screws use SCFM or ACFM?
Rotary screw data sheets list delivered flow at standard conditions (SCFM or FAD) at a stated full-load pressure. Airends are physically sized on inlet flow (ICFM), which is why intake conditions and filter condition affect real output.
The short version of SCFM vs CFM: a CFM with no S and no pressure is not a spec, it is an advertisement. Size on delivered SCFM at your working pressure, correct it for where the compressor lives, and you will stop buying air you never get.
