CFM Meaning: What CFM, SCFM and ACFM Mean, and How to Convert to L/min
HCEM Engineering TeamShare
CFM stands for cubic feet per minute: the volume of air or gas a compressor, vacuum pump or fan moves each minute. One CFM is 28.3168 litres per minute (L/min), or 1.699 cubic metres per hour (m³/h). On its own, though, a CFM figure is incomplete. A cubic foot of air holds more or less air depending on its pressure and temperature, which is why datasheets add a letter (SCFM, ACFM) or a pressure ("CFM @ 90 psi").
This guide explains what each version of CFM means, how to convert CFM to L/min and m³/h, and why the CFM of a compressor falls as pressure rises and the CFM of a vacuum pump falls as the vacuum deepens. The examples use datasheet values from our own oil-free piston pumps.
What does CFM mean?
CFM is a volumetric flow rate: cubic feet of air per minute. One cubic foot is a cube 0.3048 m on each side, which is 28.3168 litres. So a pump rated at 1 CFM moves 28.3168 litres of air every minute.
The unit comes from US and British imperial practice, and it is still widely used on compressors, air tools and vacuum pumps sold in North America and in the UK workshop market. Engineers in Europe and Asia more often write L/min, m³/h or m³/min. All of them measure the same thing: volume per unit of time.
What CFM does not tell you on its own is how much air (mass) is in that volume. Squeeze a cubic foot of air to 7 bar gauge and it holds about eight times as much air as a cubic foot at atmospheric pressure. That is the reason for SCFM, ACFM and free air delivery, covered below.
How do you convert CFM to L/min and m³/h?
The conversions are exact multiplications:
- CFM to L/min: multiply by 28.3168. 1 CFM = 28.3168 L/min.
- L/min to CFM: multiply by 0.0353 (or divide by 28.3168). 1 L/min = 0.0353 CFM.
- CFM to m³/h: multiply by 1.699. 1 CFM = 1.699 m³/h.
- m³/h to CFM: multiply by 0.5886. 1 m³/h = 0.5886 CFM.
- m³/min to CFM: multiply by 35.31. Larger industrial compressors in the UK and Europe are often rated in m³/min.
- L/s to CFM: multiply by 2.119.
Quick reference, CFM to metric:
| CFM | L/min | m³/h |
|---|---|---|
| 0.5 | 14.16 | 0.85 |
| 1 | 28.32 | 1.70 |
| 2 | 56.63 | 3.40 |
| 3 | 84.95 | 5.10 |
| 5 | 141.58 | 8.50 |
| 10 | 283.17 | 16.99 |
And the other way, L/min to CFM:
| L/min | CFM | m³/h |
|---|---|---|
| 10 | 0.353 | 0.6 |
| 25 | 0.883 | 1.5 |
| 50 | 1.766 | 3.0 |
| 100 | 3.531 | 6.0 |
| 200 | 7.063 | 12.0 |
| 500 | 17.657 | 30.0 |
For any other value, use our CFM to L/min converter. One rule matters more than the arithmetic: convert like with like. Converting an SCFM figure gives standard litres per minute, and an ACFM figure gives actual litres per minute. The unit conversion does not change the basis.
What is the difference between CFM, SCFM and ACFM?
All three are cubic feet per minute. They differ in the conditions the cubic foot is measured at.
- CFM on its own is a volume per minute with no conditions stated. On a compressor or vacuum pump datasheet it usually means the flow at the inlet, at atmospheric pressure, but you should check.
- ACFM (actual cubic feet per minute) is the volume at the real conditions at a stated point: the actual pressure, temperature and humidity there. For a compressor this is normally the inlet. Air at the outlet of the same compressor takes up far less volume, so an ACFM figure is only meaningful with its location.
- SCFM (standard cubic feet per minute) is the flow converted to a fixed set of reference conditions. Because every SCFM figure is put on the same basis, it effectively measures the quantity of air, and two SCFM figures on the same basis can be compared directly.
The catch is that "standard" is not one standard. Different industries and documents use different reference conditions:
| Basis | Pressure | Temperature | Humidity | Where you meet it |
|---|---|---|---|---|
| "Standard air" (ASME, common in US compressor usage) | 14.7 psia (1.013 bar abs) | 68 °F (20 °C) | 36% RH | US compressor and blower literature quoting SCFM |
| ISO 1217 / DIN 1945 reference inlet conditions (as commonly quoted) | 1 bar abs (14.5 psia) | 20 °C | 0% (dry) | Compressor free air delivery figures from European and Asian manufacturers |
| "Normal" conditions (DIN 1343), Nm³/h or Nl/min | 1.01325 bar abs | 0 °C | dry | Gas supply, process and industrial gas work |
| ANR reference atmosphere (ISO 8778) | 100 kPa abs | 20 °C | 65% RH | Pneumatic components such as valves and cylinders |
The differences are not trivial. The same quantity of air takes up about 7% more volume at a 20 °C reference than at 0 °C (293.15 ÷ 273.15 = 1.073), and about 1.3% more at 1 bar than at 1.013 bar. A supplier's figure and your requirement should therefore be on the same basis before you compare them. If a datasheet does not say which basis it uses, ask; the edition of the standard and the test method also matter.
How do you convert ACFM to SCFM?
For dry air, the ideal gas law gives a good first estimate:
SCFM = ACFM × (Pactual ÷ Pstandard) × (Tstandard ÷ Tactual)
Both pressures must be absolute (gauge pressure plus atmospheric pressure) and both temperatures absolute (°C + 273.15). Two examples, using 1.013 bar abs and 20 °C as the standard:
- Compressed air. 1 ACFM measured at 7 bar gauge and 20 °C is at 8.013 bar abs. SCFM = 1 × (8.013 ÷ 1.013) ≈ 7.9 SCFM. A small volume of compressed air holds a lot of free air.
- Vacuum. 10 ACFM measured at −92 kPa gauge (about 9.3 kPa abs) and 20 °C is 10 × (9.3 ÷ 101.3) ≈ 0.92 SCFM. A large volume of rarefied gas holds very little air.
Altitude works the same way. At 1,500 m the standard atmosphere is about 84.6 kPa, so each cubic foot a compressor draws in there holds about 17% less air than at sea level. If the machine will run high up or in hot conditions, see our guide to altitude and ambient conditions in pump selection. For moist air, a precise correction also removes the water vapour's share of the pressure; for most selection work the dry formula is close enough.
What is free air delivery (FAD), and why is it lower than displacement?
Compressor datasheets use two very different "CFM" figures, and confusing them is the most common mistake when comparing machines.
- Displacement (also called swept volume or piston displacement) is a calculation: the volume the pistons sweep per stroke, multiplied by the strokes per minute. It is what the compressor would deliver if it were perfect.
- Free air delivery (FAD) is a measurement: the compressed air actually delivered at the outlet, at a stated discharge pressure, converted back to the conditions at the inlet. ISO 1217 sets out how it is measured.
FAD is always lower than displacement, for physical reasons:
- Clearance volume. A piston cannot push out every last bit of air. The compressed air left at the top of the stroke re-expands on the way down, and fresh air only enters once it has, so part of each stroke is lost.
- Valve and inlet losses. Air has to be drawn through filters and valves, so the cylinder fills at slightly below atmospheric pressure.
- Heating. The cylinder warms the incoming air, and warm air is less dense.
- Leakage past piston rings and valves.
All of these get worse as discharge pressure rises, so the gap between displacement and FAD widens at higher pressure. Some workshop compressors quote both figures, with the larger displacement number in the product name and the FAD further down the specification. Compare FAD with FAD, at the same pressure. Our guide to high-CFM oilless pump spec sheets goes through the other lines on a datasheet.
What does "CFM @ 90 psi" mean?
It means the flow the compressor delivers while holding a discharge pressure of 90 psi (6.2 bar). A compressor's flow is always tied to the pressure it is pushing against, so a flow figure without a pressure is only half a specification. Many air tools in North America are rated at 90 psi, which is why US compressors often list "SCFM @ 40 psi" and "SCFM @ 90 psi" side by side, the second figure always the lower one.
Why does CFM fall as pressure rises? At a higher discharge pressure each stroke has to compress the air further before the outlet valve opens, more air is left in the clearance volume to re-expand, more leaks past the seals, and the motor works harder. The result is a flow curve that slopes down as pressure goes up.
You can see the effect in our own range. Two variants of our HC100 oil-free air pump are built for different jobs: the HC100-30/2 is rated at ≥30 L/min (1.06 CFM) at 200 kPa (2 bar, 29 psi), while the HC100-9/7 is rated at 9 L/min (0.32 CFM) at 700 kPa (7 bar, 102 psi). They are different builds, not one pump measured twice, but the pattern is the one every compressor follows: the higher the pressure, the lower the flow.
When a specification says "at 90 psi", check whether it means the compressor's outlet or the point of use. Hoses, fittings, filters and dryers all cost pressure on the way, so the compressor has to deliver its flow at a slightly higher pressure than the tool needs.
Why does a vacuum pump's CFM fall as the vacuum deepens?
A vacuum pump's CFM rating is normally its free-air flow: the flow with the inlet open to the atmosphere. That is the best case. On every stroke the pump traps roughly the same volume, but as the pressure in the chamber falls, that volume holds fewer molecules. At −92 kPa gauge a cubic foot of gas holds about 9% of the air it held at atmospheric pressure, so each stroke removes less. Near the pump's ultimate vacuum, the useful flow approaches zero.
That is also why the high-vacuum versions of our piston pumps show a lower flow. The HC580A is rated at 120 L/min (4.24 CFM) with an ultimate vacuum of −92 kPa, while its high-vacuum version reaches −98 kPa at 60 L/min (2.12 CFM). Depth costs flow.
For the mechanism inside the pump, see how vacuum pumps work. To check what a pump really delivers on your system, see how to test a vacuum pump.
CFM vs PSI: what is the difference?
They measure two different things, and you need both.
- CFM is how much air moves: quantity per minute.
- PSI (pounds per square inch; 1 bar = 14.5 psi) is how hard it pushes: pressure.
A blow gun at 2 bar can use a lot of CFM, while a pneumatic actuator at 8 bar may need very little. You cannot convert one into the other: there is no "CFM to PSI" formula, only a compressor's flow curve linking the two for that machine. Pressure decides whether a device works at all; flow decides whether the compressor can keep up with it. A third number, duty cycle, decides how long it can keep doing so. For a longer comparison, read PSI vs CFM explained.
How much air do HCEM pumps move in CFM?
Our datasheets give flow in L/min, at a stated pressure for compressors and with the ultimate vacuum for vacuum pumps. Here are five models converted to CFM:
| Model | Type | Rated flow | In CFM | Condition as stated |
|---|---|---|---|---|
| HC100-30/2 | Oil-free air pump, AC | ≥30 L/min | 1.06 | at 200 kPa (2 bar, 29 psi) |
| HC100-9/7 | Oil-free air pump, AC | 9 L/min | 0.32 | at 700 kPa (7 bar, 102 psi) |
| HC1100 | Oil-free air pump, AC | 100 L/min | 3.53 | at 800 kPa (8 bar, 116 psi) |
| HC580A | Oil-free vacuum pump, AC | 120 L/min, or 60 L/min | 4.24, or 2.12 | free-air flow; ultimate vacuum −92 kPa, or −98 kPa (high-vacuum version) |
| HC1500A | Oil-free vacuum pump, AC | 210 L/min, or 110 L/min | 7.42, or 3.88 | free-air flow; ultimate vacuum −92 kPa, or −98 kPa (high-vacuum version) |
Two things to note when you compare these with other suppliers' figures. A compressor flow "at 800 kPa" is a delivered flow at that pressure, not a displacement figure, so compare it with the other supplier's flow at the same pressure. A vacuum pump's free-air flow is its best case, and the flow at your working vacuum will be lower. Browse the full ranges of AC air compressor pumps, DC air compressor pumps and oil-free vacuum pumps.
Knowing what CFM means is the first step; choosing the flow your machine needs is the next. Our guide on how to size an oil-free air pump by CFM, PSI and duty cycle walks through that calculation.
Common questions
How many L/min is 1 CFM?
1 CFM is 28.3168 L/min. For a quick mental estimate, 1 CFM is a little over 28 L/min, and 100 L/min is about 3.5 CFM.
Can you convert CFM to PSI?
No. CFM is flow and PSI is pressure, so there is no conversion between them. What links them is a specific compressor's performance: its datasheet or flow curve tells you how many CFM it delivers at each pressure.
What is the difference between L/min, Nl/min and SLPM?
L/min is litres per minute with no conditions stated, or at actual conditions. Nl/min (normal litres per minute) is referenced to "normal" conditions, usually 0 °C and 1.013 bar. SLPM (standard litres per minute) is referenced to a "standard", which varies by manufacturer: 0 °C, 20 °C and 25 °C are all common. Check the reference before converting SLPM to SCFM.
What does CFM mean on a vacuum pump?
It is usually the free-air flow, measured with the inlet at atmospheric pressure. It tells you how fast the pump starts to evacuate a volume, not how fast it works at depth. The ultimate vacuum, given separately, tells you how deep it can go.
Is a higher CFM always better?
Only if the extra flow is available at the pressure or vacuum you need. A pump with a high free-air or low-pressure rating can deliver less than a smaller-looking pump at your working point. Compare flow at the same condition, then check power, noise and duty cycle.
Use our CFM to L/min converter for quick conversions, or send us your flow, pressure or vacuum level, supply voltage and duty cycle, and our engineers will recommend a model, including OEM versions built for your machine.