ACFM to SCFM Calculator

Concepts behind this calculation

Convert airflow between actual and explicitly specified reference air conditions using moist-air specific volume and a conserved dry-air mass flow.

Actual CFM describes volume at an operating air state, while reference SCFM describes volume at an explicitly defined reference state. Temperature, absolute pressure and humidity determine the specific volume at each state. Here specific volume means mixture volume per unit dry-air mass, so multiplying source flow by the ratio of target to source specific volume preserves dry-air mass flow.

The word standard does not supply a unique temperature, pressure or humidity convention. Those conditions must come from the applicable instrument or project source. When the two states have different humidity ratios, this conversion does not conserve total moist-air mass. Duct static pressure is also not the absolute barometric pressure needed for either state; substituting it would change the physical meaning of the calculation.

How to use this calculator

1. Select Conversion direction and enter Source airflow on that direction’s source-state basis.
2. Enter Actual dry-bulb temperature, Actual total absolute pressure and Actual relative humidity, then the three corresponding Reference conditions.
3. Complete Reference-condition source or description and Reference-condition confirmation. Review both specific volumes and the dry-air mass flow with the converted result.

Calculation method and applicability

pv = RH/100 × pws(T); W = 0.621945 × pv/(pabs−pv); v = 0.370486 × (T°F+459.67) × (1+1.607858W)/pabs[psia]; Qtarget = Qsource × vtarget/vsource; mdry-air = 60Qsource/vsource.

W = humidity ratio (lb water/lb dry air); v = specific volume (ft³/lb dry air); pabs = total absolute pressure (psia); T = °F; Q = actual or reference flow (CFM as labeled); mdry-air = conserved dry-air mass flow (lb/h).

Input definitions and units

Conversion direction

Choose whether the source flow is actual ACFM or SCFM at the reference state you enter below. Choices: Actual ACFM to reference SCFM; Reference SCFM to actual ACFM.

Source airflow (CFM at source state)

Enter the volume flow at the state selected by the conversion direction. It must be positive and finite.

Actual dry-bulb temperature (°F)

The actual state for ACFM. This calculator supports −40 to 140°F.

Actual total absolute pressure (psia)

Use local total barometric absolute pressure. Do not enter duct static pressure or gauge pressure.

Actual relative humidity (%)

Enter the actual-state RH from 0 to 100 percent.

Reference dry-bulb temperature (°F)

Enter the temperature required by your applicable SCFM convention or project specification; no standard temperature is assumed.

Reference total absolute pressure (psia)

Enter the reference total absolute pressure, not gauge or duct static pressure.

Reference relative humidity (%)

Enter the reference-state RH required by your stated convention, from 0 to 100 percent.

Reference-condition source or description

Name the specification, instrument, project requirement or other source that establishes all three reference conditions. This note is not independently authenticated.

Reference-condition confirmation

Confirm that the entered reference temperature, absolute pressure and RH match the applicable source before calculating. Choices: Not confirmed; I confirmed the stated reference conditions.

How to interpret the result

Carry the converted flow together with its reference description, airflow ratio, humidity ratios and dry-air mass basis. Use actual-state flow in downstream tools that require ACFM rather than substituting the reference volume unchanged.

Worked examples

Assumptions and limits

• SCFM is evaluated at the explicitly entered reference temperature, total absolute pressure and RH. Different standards, instruments and contracts may define different reference states; the calculator does not choose or certify one.
• The conversion conserves dry-air mass flow using moist-air specific volume per unit dry-air mass. It is not a total moist-air mass-flow conversion when humidity ratio changes between the two entered states.
• Uses perfect-gas moist-air relations and ASHRAE 2017 Hyland-Wexler saturation pressure coefficients over −40 to 140°F. Real-gas effects, contaminants, gas mixtures other than ordinary moist air, compressibility-meter factors and instrument calibration are outside scope.
• Both pressures are total absolute barometric pressures. Duct static pressure, fan pressure and gauge pressure are not valid substitutes. No elevation or weather lookup is performed.
• Each state must have relative humidity from 0 to 100 percent and water-vapor partial pressure below total absolute pressure. No duct sizing, fan selection, code compliance or field measurement certification is calculated.

Sources

• ASHRAE Handbook—Fundamentals 2017, IP Chapter 1: PsychrometricsEquations 7–8 define humidity ratio; Equations 24–26 define moist-air specific volume on a dry-air mass basis and the perfect-gas relation used here. The chapter notes the perfect-gas approximation and its applicability limits.
• ASHRAE Handbook—Fundamentals 2017, SI Chapter 1Hyland-Wexler ice/water saturation-pressure coefficient source with a triple-point branch. Coefficients are converted consistently to English-unit inputs and Pa vapor pressure.
• UFC 3-420-02: Compressed Air, Appendix A-4Directly states the I-P ACFM/SCFM relation with absolute pressure, moisture vapor pressure and absolute Rankine temperature. Its compressed-air context is not an HVAC duct design or code rule; this tool generalizes the dry-reference relation through ASHRAE moist-air specific volume at both user-entered states.