What this calculator does
SCFM, standard cubic feet per minute, restates an airflow measurement as if it had been taken at one fixed reference temperature and pressure, rather than whatever conditions actually existed at the time. This matters because air expands when it is hot and contracts under low pressure, so the same mass of air moving through a system reads as a different volumetric flow (ACFM, actual cubic feet per minute) depending on where and when it was measured.
Compressor and equipment ratings are almost always quoted in SCFM precisely so two pieces of equipment, or the same equipment tested on two different days, can be compared fairly. This calculator takes an actual airflow reading along with the actual temperature and pressure it was taken at, and converts it to the equivalent standard airflow.
The formula
The conversion scales actual airflow by the ratio of actual pressure to standard pressure, and by the ratio of standard temperature to actual temperature, with both temperatures converted to an absolute scale (Rankine, degrees Fahrenheit plus 459.67) so the ratio behaves correctly. Standard conditions are fixed here at 68°F and 14.696 psia, a common industry reference point.
| Term | Meaning |
|---|---|
| ACFM | Actual cubic feet per minute: the airflow as measured at the real, current temperature and pressure. |
| SCFM | Standard cubic feet per minute: the same flow restated at fixed standard conditions, for fair comparison. |
| Rankine | An absolute temperature scale used in the correction so that doubling absolute temperature genuinely doubles the ratio; degrees Fahrenheit alone would give the wrong answer. |
The inputs explained
| Field | What to enter |
|---|---|
| Actual airflow (ACFM) (ft³/min) | The airflow reading as actually measured, in actual cubic feet per minute. |
| Actual air temperature (°F) | The air temperature at the point and time the ACFM reading was taken. |
| Actual air pressure (psia) | The absolute air pressure at the point and time the ACFM reading was taken. |
When to use it
Comparing a compressor rating to a site reading
A compressor nameplate rated in SCFM can be checked against a field ACFM measurement once that measurement is converted to the same standard basis.
Sizing equipment for a hot or high-altitude site
A site that runs hotter or at lower ambient pressure than standard conditions delivers less SCFM for the same ACFM, which matters when equipment is specified by its standard-rated capacity.
Checking a reading taken on a very cold day
Cold, dense air pushes more mass through the same actual volumetric flow, so an ACFM reading taken in winter converts to a higher SCFM than the same ACFM reading taken in summer.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does actual temperature affect standard airflow?
The same 100 ACFM reading, taken at different actual air temperatures.
How does ACFM scale to SCFM at typical shop conditions?
A near-standard actual temperature and pressure, so the correction stays small, across a range of flow readings.
Questions
Why is SCFM lower than ACFM on a hot day?
Hot air is less dense, so a given actual volumetric flow carries less mass of air than the same volumetric flow would at the cooler standard temperature. Converting to standard conditions scales the reading down to reflect that lower mass flow.
What standard conditions does this calculator use?
It uses 68°F and 14.696 psia, a widely used industry reference point for compressed air work. Other standards exist (some use 60°F, others 20°C), so always check which standard a nameplate rating or specification is quoting against.
Does altitude affect this calculation?
Yes, through the actual pressure input. Higher altitude sites have lower ambient atmospheric pressure, which lowers actual pressure and therefore lowers the resulting SCFM for the same ACFM reading.
How is this different from the CFM airflow calculator?
The CFM airflow calculator works out actual airflow (ACFM) from duct dimensions and velocity, or from room volume and air changes per hour. This calculator takes that actual airflow figure a step further, converting it to the standard-conditions SCFM figure used for rating and comparing equipment.
To work out the actual airflow figure in the first place, from duct size and velocity or from room volume and air changes, see the CFM airflow calculator.