In the United States, the EPA defines standard conditions for volume and volumetric flow as a temperature of 293 K (68 °F) and a pressure of 101.3 kilopascals (29.92 in.
Actual cubic foot per minute (ACFM) is the volume of gas flowing anywhere in a system, taking into account its temperature and pressure.
If the system were moving a gas at exactly the "standard" condition, then ACFM would equal SCFM.
When positive pressure is applied to a standard cubic foot of gas, it is compressed.
When a vacuum is applied to a standard cubic foot of gas, it expands.
Cubic feet per minute (CFM) is an often confusing term because it has no single definition that applies to all instances.
This means that the air velocity in a system is the same even though mass flow rate through the fan is not.