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Manufacturers of heating and cooling equipment often specify air‑flow capacity in Cubic Feet per Minute (CFM). Because air density varies with temperature and pressure, the same device can deliver different CFM values under different conditions. To standardize comparisons, the industry also reports capacity in Standard Cubic Feet per Minute (SCFM), which assumes fixed reference conditions.

What Are CFM and SCFM?

CFM, or Actual Cubic Feet per Minute (ACFM), represents the true volumetric flow of air at the device’s operating temperature and pressure. SCFM, on the other hand, is expressed at a fixed set of standard conditions that remove this variability.

According to the American Society of Mechanical Engineers (ASME), the standard reference values are: atmospheric pressure = 14.7 psi, room temperature = 68 °F (528 °R), relative humidity = 36 %, and air density = 0.075 lb/ft³. When a unit’s capacity is listed in SCFM, these are the conditions assumed.

Converting Between SCFM and ACFM

The ideal gas law, pV = nRT, links pressure, volume, and temperature. While air is not a perfect ideal gas, treating it as such provides a practical conversion. By expressing density (ρ) as P/(RT) and noting that mass flow m/t = ρ·(V/t), we derive the following relationship:

SCFM = ACFM × (PA/PS) × (TS/TA)

Here, PA = actual pressure, PS = standard pressure (14.7 psi), TA = actual temperature, and TS = standard temperature (528 °R). Rearranging gives:

ACFM = SCFM × (PS/PA) × (TA/TS)

These formulas let you adjust a rating from standard to the conditions you actually care about, or vice versa.

Accounting for Humidity

For applications where moisture content significantly impacts performance, a more refined equation incorporates humidity:

ACFM = SCFM × PS – [(RHS × PVS)/Pb] – [(RHA × PVA) × (TA/TS) × (Pb/PA)]

In this expression: RHS = standard relative humidity, RHA = actual relative humidity, PVS = saturated vapor pressure at standard temperature, PVA = saturated vapor pressure at actual temperature, and Pb = barometric pressure.

Using the humidity‑adjusted formula ensures the most accurate comparison for HVAC systems operating in environments with varying moisture levels.