Within the air compressor industry, acronyms are found in abundance, especially when it comes to measuring airflow. The most common ones are SCFM and ACFM, with ICFM coming right after them. In order to understand these, you must first understand CFM, cubic feet per minute, which is the general measurement for the amount of air that a compressor provides.

If you require a refresher or more information on CFM, visit our What Is CFM and What Does CFM Mean on An Air Compressor guide. This article will focus on providing you with all the relevant information on SCFM, ACFM & ICFM, as well as presenting how to calculate and convert each value.

Table of Contents

What is SCFM?

SCFM, standard cubic feet per minute, is a standard measurement of air output that takes atmospheric conditions into account. The SCFM of an air compressor is a specific set of parameters determined by ASME (the American Society of Mechanical Engineers) and is recognized not only in the compressed air industry but many others.

The SCFM calculation is based on standard atmospheric conditions in order to measure the mass flow of air coming from an air compressor. The standard conditions include:

  • atmospheric pressure of 14.696 PSIA
  • relative humidity of 36%
  • ambient temperature of 68°F

It is possible to calculate the maximum SCFM output of an air compressor by using these conditions. Essentially, SCFM is the one and only way to compare air compressors across the industry.

What is ACFM?

The ACFM, or actual cubic feet per minute, typically has different definitions depending on the industry which can cause a lot of confusion. Within, the air compressor industry, ACFM is the true air mass flow given during a set of specific real-life conditions.

It can be seen as a measurement of CFM based on the actual output of air in a certain environment. And so, it can be quite tricky because surrounding environments and atmospheric conditions impact it, and so the ACFM of a compressor will vary in different locations.

For instance, an air compressor situated high above sea level compared to one at sea level will have a significantly lower output due to the ambient conditions so high up.

What is the Difference Between ACFM & SCFM

At standard air conditions, where there are no efficiency losses, SCFM and ACFM are equal to one another. Where the inlet conditions vary from the standard conditions, consideration must be taken to ensure that the specified air compressor has enough power to generate the adequate air mass flor for optimum performance of the connect pneumatic tool.

ACFM will typically be a higher number than SCFM if any of the following conditions are different from the standard conditions listed further up this page:

  • atmospheric pressure is lower (greater elevation from sea-level)
  • humidity is higher
  • temperature is higher

A good way to think about this is that a compressor needs to work harder the further it drifts away from standard conditions. And therefore, a more powerful air compressor with a higher SCFM rating would be required to generate the desired ACFM for the air tool to perform effectively.

Here are some quick calculations that can be used as a guideline:

  • For every 1000 ft, or 305m, of elevation above sea level (standard condition), ACFM demand increases by approx. 5%
  • For every 20% of humidity increase over 36% (standard conditions), ACFM demand increases by approximately .5%.
  • For every 20°F (11.1°C) of ambient temperature increase over 68°F (standard conditions), ACFM demand increases by approximately 5%.

What is ICFM?

ICFM, inlet cubic feet per minute, identifies the flow rate at the inlet of an air compressor after the inlet filter, blower, or booster. As air passes through these components, there will typically be pressure drop or rise, if there wasn’t then the ICFM and ACFM values would be the same.

The greater the difference in pressure and temperature through the components, the greater the difference between ICFM and ACFM.

Calculating & Converting SCFM, ACFM & ICFM

When it comes to calculating, or converting these values, it can be done by following the process below. SCFM of air compressors is provided to you already when you purchase an air compressor, typically referred to as the CFM output of the compressor.

If you wish to calculate the ACFM, you can convert the SCFM value using the following formula:

ACFM = SCFM x [Pstd / (Pact – Psat Φ)] x (Tact / Tsat)

Where:
ACFM = actual cubic feet per minute
SCFM = standard cubic feet per minute
Ptsd = standard absolute pressure (psia)
Pact = absolute pressure at actual level (psia)
Psat = saturation pressure at actual temperature (psi)
Φ = actual relative humidity
Tact = actual ambient air temperature (oR)
Tsat = standard temperature (oR)

To then calculate ICFM, you must convert ACFM using the following formula:

ICFM = ACFM x (Pact / Pf) x (Tf / Tact)

Where:

ICFM = inlet cubic feet per minute
Pf = pressure after filter or inlet equipment (psia)
Tf = temperature after filter or inlet equipment (oR)

When it comes to the temperatures in both sets of equations, they’re standard temperatures in oR. So, for example, at a temperature of 50oF, you would implement 510oR (50 + 460).

SCFM, ACFM, or ICFM

The most difficult tasks is sizing a compressor properly, or specifying the compressor’s reduced capacity. It is thereby important to note that the proper understanding of these terms will help when selecting a compressor. SCFM should be used to compare the differences in compressors capacities, ACFM for actual non-standard site conditions and proper load applications while, ICFM should be used when a filter, booster or blower is added into the system.

At higher altitudes, you must account for the decrease in pressure when estimating a compressor’s performance. Sometimes nlowers or boosters are used for economic reasons but they are not always the best option to use. Therefore, when tighter controls are employed, ACFM should not be used to determine a compressors capacity, ICFM should be.

FAQs (Frequently Asked Questions)

How is ICFM calculated?

ICFM is calculated by multiplying the ACFM (actual ) by the absolute pressure at actual level (psia) divided by pressure after filter or inlet equipment (psia) and then multiplied again by the temperature after filter or inlet equipment (oR) divided by the actual ambient air temperature (oR).

Is ACFM the same as SCFM?

ACFM is only the same as SCFM if the compressor is under standard conditions and there are no efficiency losses. The standard conditions include atmospheric pressure at sea-level of 14.696 PSIA, relative humidity of 36%, and an ambient temperature of 68°F (19°C).

How is SCFM different from CFM?

The key difference between SCFM and CFM is that SCFM measures the CFM of a compressor based on ideal, or standard, conditions. While CFM is a measure of the actual flow rate. CFM is typically the recognized value across the United States for measuring air flow.

How do I convert SCFM to ACFM?

To convert SCFM to ACFM you must multiply SCFM by the standard absolute pressure (psia) divided by the absolute pressure at actual level (psia) minus the saturation pressure at actual temperature (psi) multiplied by actual relative humidity. And then, multiply this by the actual ambient air temperature (oR) divided by standard temperature (oR).


If you have any questions regarding SCFM vs ACFM vs ICFM and their differences, please leave a comment below, with a photo if applicable, so that someone can help you!