Air-Compressor CFM ↔ SCFM Calculator
Convert between actual and standard cubic feet per minute for compressed air systems.
Conversion Parameters
Conversion Results
Notes & Definitions
- CFM (Cubic Feet per Minute): Actual volume flow rate at operating conditions.
- SCFM (Standard Cubic Feet per Minute): Volume flow rate normalized to standard conditions (14.7 psia, 68°F, 0% humidity).
- Atmospheric Pressure: Decreases with altitude, affecting air density.
- Absolute Pressure: Gauge pressure plus atmospheric pressure.
- Temperature Effect: Higher temperatures expand air, reducing density.
- Humidity Effect: Water vapor displaces air, reducing effective air density.
How this CFM ↔ SCFM calculator works
Everything below describes the exact method the tool above runs in your browser, so its results can be reproduced by hand or checked against a datasheet.
CFM, ACFM and SCFM defined
- CFM (cubic feet per minute)
- A volumetric flow rate: how many cubic feet of gas pass a point each minute. On its own the term does not say at what pressure or temperature the volume was measured, which is why compressor and tool ratings are easy to misread. In this tool, "Actual CFM" means ACFM.
- ACFM (actual cubic feet per minute)
- The real volume flowing at the operating pressure, temperature and humidity of the point you are looking at. Air at 100 psig occupies far less volume than the same air at atmospheric pressure, so an ACFM figure is always tied to its conditions.
- SCFM (standard cubic feet per minute)
- The same mass flow expressed as the volume it would occupy at a defined set of standard conditions. It is effectively a mass-flow figure, which is what lets equipment be compared on equal terms. There is no single universal standard: commonly used definitions include 14.7 psia, 68 °F and 36 % relative humidity (ASME) and 14.5 psia (1 bar absolute), 68 °F and dry air (ISO 1217, used by CAGI). Check which definition a datasheet uses before comparing numbers.
Reference conditions used by this tool
This calculator standardises to the following fixed values, taken directly from its code:
| Quantity | Value used | Note |
|---|---|---|
| Standard pressure | 14.7 psia | Also used as the sea-level ambient pressure (altitude 0 ft). |
| Standard temperature | 528 °R | Nominally 68 °F. The exact Rankine value of 68 °F is 527.67 °R, so an input of exactly 68 °F gives a temperature factor of 528 / 527.67 = 1.0006, not 1.0000. |
| Reference humidity | 0 % RH (dry air) | The humidity factor equals 1 only at 0 % relative humidity. |
| Ambient pressure model | 14.7 × (1 − 6.8754 × 10⁻⁶ × altitude in ft)^5.2559 | Standard-atmosphere barometric approximation; it ignores the actual barometer reading on the day. |
| Humidity model | Linear, 2 % maximum reduction at 100 % RH | A simplification. A vapour-pressure model would vary with temperature and pressure. |
Because the standard temperature constant is 528 °R, this tool's "no change" point is 0 psig, 68 °F, 0 ft and 0 % RH, where the conversion factor is 1.0006. The default inputs shown when the page loads (100 psig, 70 °F, 60 % RH) are operating conditions, not the standard.
The formula this calculator uses
The tool builds one conversion factor K from four corrections and multiplies or divides by it. The steps below are exactly those in the page script.
P_atm = 14.7 × (1 − 6.8754 × 10⁻⁶ × h)^5.2559gives the ambient pressure in psia from the altitude h in feet.P_abs = P_g + P_atmgives the absolute pressure in psia from the gauge pressure P_g in psig.F_T = 528 / (T_F + 459.67)is the temperature factor; T_F + 459.67 is the operating temperature in degrees Rankine.F_H = 1 − 0.02 × (RH / 100)is the humidity factor from the relative humidity RH in percent.K = (P_abs / 14.7) × F_T × F_His the conversion factor.- CFM → SCFM:
Q_s = Q_a × K. SCFM → CFM:Q_a = Q_s × (14.7 / P_abs) / F_T / F_H, which is the same asQ_s / K.
The tool shows the result and the two pressures to two decimal places, and the conversion factor and temperature factor to four. In SCFM → CFM mode the "Conversion Factor" it displays is 1 / K.
| Symbol | Meaning | Unit |
|---|---|---|
| Q_a | Actual flow (ACFM, entered as "Actual CFM") | ft³/min at operating conditions |
| Q_s | Standard flow (SCFM) | ft³/min at 14.7 psia, 528 °R, dry air |
| P_g | Operating gauge pressure | psig |
| h | Altitude above sea level | ft |
| P_atm | Ambient (atmospheric) pressure at altitude h | psia |
| P_abs | Absolute operating pressure | psia |
| T_F | Operating temperature | °F (converted to °R inside the formula) |
| RH | Relative humidity at operating conditions | % |
| F_T, F_H, K | Temperature factor, humidity factor, conversion factor | dimensionless |
Worked examples (checked against the tool's code)
Both examples state every condition they assume. The figures are what the calculator above returns for the same inputs, so you can enter them and compare.
Example 1: CFM → SCFM at the tool's default inputs
Declared conditions: 100 ACFM, 100 psig, 70 °F, 0 ft (sea level), 60 % RH.
- P_atm = 14.7 × (1 − 0)^5.2559 = 14.70 psia
- P_abs = 100 + 14.70 = 114.70 psia
- F_T = 528 / (70 + 459.67) = 528 / 529.67 = 0.9968
- F_H = 1 − 0.02 × 0.60 = 0.988
- K = (114.70 / 14.7) × 0.9968 × 0.988 = 7.8027 × 0.9968 × 0.988 = 7.6848
- Q_s = 100 × 7.6848 = 768.48 SCFM
Reading: 100 ft³/min of air measured at 100 psig and 70 °F carries as much air as 768.48 ft³/min would at this tool's standard conditions.
Example 2: SCFM → CFM at altitude
Declared conditions: 250 SCFM required, 90 psig, 95 °F, 5,000 ft above sea level, 40 % RH.
- P_atm = 14.7 × (1 − 6.8754 × 10⁻⁶ × 5000)^5.2559 = 14.7 × 0.96562^5.2559 = 12.23 psia
- P_abs = 90 + 12.23 = 102.23 psia
- F_T = 528 / (95 + 459.67) = 528 / 554.67 = 0.9519
- F_H = 1 − 0.02 × 0.40 = 0.992
- Displayed factor = (14.7 / 102.23) / 0.9519 / 0.992 = 0.1523 (this is 1 / K, with K = 6.5671)
- Q_a = 250 × 0.1523 = 38.07 CFM
Reading: delivering 250 SCFM at 90 psig, 95 °F and 5,000 ft means only 38.07 ft³/min actually passes through the pipe, because the air is compressed. The thinner air at altitude lowers P_abs, and with it K, so slightly more actual volume is needed than at sea level.
Common errors when converting CFM and SCFM
- Using gauge pressure as absolute pressure. A gauge reads zero at atmospheric pressure, but the formula needs absolute pressure, so the ambient pressure must be added: 100 psig at sea level is 114.7 psia. Treating a 100 psig reading as 100 psia would use 100 / 14.7 instead of 114.7 / 14.7 and understate SCFM by about 13 %.
- Using °F or °C directly in the temperature ratio. Gas-law ratios only work with absolute temperature. Convert °F to Rankine (add 459.67) or °C to Kelvin (add 273.15) first. This tool takes Fahrenheit and performs the Rankine conversion for you.
- Ignoring altitude or the local barometer. Ambient pressure falls with altitude, which changes P_abs even at the same gauge reading. This tool estimates ambient pressure from altitude with a standard-atmosphere formula; it does not know the weather on the day, so use a measured barometric pressure when accuracy matters.
- Mixing humid-air and dry-air assumptions. Water vapour displaces air, so humid air carries less air per cubic foot. This tool's reference is dry air and its humidity correction is a simplified linear 2 % maximum. Datasheets that standardise to 36 % RH (ASME) will not match a dry-air figure exactly.
- Comparing SCFM figures from different standards. References of 14.7 psia and 68 °F, 14.5 psia and 68 °F, or 14.73 psia and 60 °F all give slightly different SCFM for the same mass flow. Confirm the reference conditions before comparing a compressor rating with a tool's demand.
A note on SCFH
SCFH is standard cubic feet per hour: the same standardised volume as SCFM, counted per hour instead of per minute. Once a flow has been converted to standard conditions, SCFH = SCFM × 60 and SCFM = SCFH ÷ 60. For Example 1, 768.48 SCFM at this tool's standard conditions is 768.48 × 60 = 46,108.8 SCFH under the same declared conditions.
There is no fixed number that turns SCFH directly into actual CFM. The ×60 or ÷60 step only changes the time base; the pressure, temperature, altitude and humidity conversion above still has to be done first, and its result depends on the conditions you declare.