ACFM (Actual Cubic Feet per Minute) vs. SCFM (Standard Cubic Feet per Minute)

The air compressor industry is full of acronyms for measuring airflow or consumption.  Two common measurements are ACFM and SCFM.

ACFM (Actual Cubic Feet per Minute) may have different definitions depending on the industry.  VMAC defines ACFM as the true air mass flow given a certain set of real life conditions.  A pneumatic tool may demand a specific minimum air mass flow for it to perform properly on the job site.  ACFM varies depending on atmospheric conditions on the job site.

SCFM (Standard Cubic Feet per Minute) is more easily defined and is recognized worldwide across many industries.  VMAC defines SCFM as the air mass flow generated at “standard conditions”.  SCFM is determined by applying ASME (American Society of Mechanical Engineers) calculations based on atmospheric conditions (“standard conditions”) to measured air mass flow from an air compressor.

SCFM “standard conditions” include: atmospheric pressure at sea-level of 14.7 PSIA (760 mmHg), relative humidity of 36%, and ambient temperature of 68oF (19oC).  After completing the calculations, the maximum SCFM output of the air compressor is revealed.  SCFM is the only way to compare “apples to apples”, or air compressor to air compressor, as operating conditions may vary depending on where the air compressor air mass flow is being measured.

At “standard conditions”, with no efficiency losses, SCFM equals ACFM.  Where inlet conditions vary from “standard conditions”, consideration may be necessary to ensure the specified air compressor has enough power to generate adequate air mass flow for the tool to perform properly on the job site.

The ACFM demand by the tool may be higher if any one of the following conditions is different from standard conditions: atmospheric pressure is lower, humidity is higher and/or temperature is higher.  Another way of thinking about this is the air compressor needs to work harder as the job site drifts away from “standard conditions”.  A more powerful air compressor (with higher SCFM rating) may be required to generate adequate ACFM for the pneumatic tool to perform properly on the job site.

The bottom line is; unless you are only using your air compressor on the beach on a cool spring day, add a buffer when specifying your equipment.

Other CFM types based solely on theoretical calculations:
ICFM (Inlet Cubic Feet per Minute) and DCFM (Displaced Cubic Feet per Minute).

If you have any questions about this article or anything mobile compressor related, please contact us.

Quick Calculations:

  • For every 1000 ft (305 m) of elevation increase over “standard conditions”, ACFM demand increases by approximately 5%.
  • For every 20oF (11.1oC) of ambient temperature increase over “standard conditions”, ACFM demand increases by approximately 5%.
  • For every 20% of humidity increase over “standard conditions”, ACFM demand increases by approximately .5%.

Example Calculation:

A 1” impact gun may have a minimum air mass flow requirement of 45 CFM.  On the job site in Ft McMurray, Alberta at an elevation of 1,211 ft (369 m) on June 22, 2011 at 2:00PM, atmospheric pressure is 14.2 PSIA (732 mmHg), humidity is 24%, and temperature is 84oF (27oC).   For an air compressor to generate 45 ACFM in these conditions, it actually needs to have an SCFM rating of approximately 49 SCFM.