SCFM to ACFM Calculator: Standard vs Actual Airflow at Your Pressure, Temperature, Altitude and Humidity
Convert SCFM to ACFM or back for compressed air at any line pressure, temperature, altitude and humidity, with six standard-air definitions and metric units.
How it works
ACFM = SCFM × [(P_std − φ_std·P_sat,std) ÷ (P_act − φ_act·P_sat,act)] × (T_act ÷ T_std) · SCFM = ACFM ÷ that same factor · P_act = gauge pressure + atmospheric pressure (absolute) · T in °R (°F + 459.67) or K · P_atm = 14.696 psia × (1 − 2.25577×10⁻⁵ × elevation in m)^5.25588
SCFM (standard cubic feet per minute) measures how much air flows as if it were at a fixed reference state; ACFM (actual cubic feet per minute) is the volume that same air really fills at the pressure and temperature in your pipe or at your intake. Because the amount of dry air does not change, the conversion follows the ideal gas law: the actual volume shrinks in proportion to absolute pressure and grows in proportion to absolute temperature. Water vapour is handled with partial pressures, so only the dry-air share of the pressure carries the standard flow: at 80% humidity and 80 °F the vapour takes up about 0.4 psi of the total. Gauge pressure is turned into absolute pressure by adding the local atmospheric pressure, which this calculator estimates from your elevation with the standard atmosphere, or takes from a barometer reading. The weak point of any SCFM figure is the word “standard”. Compressor makers, gas suppliers and standards bodies use different reference states, such as 14.7 psia and 68 °F dry air, 60 °F for the US gas industry, ASME “standard air” with 36% humidity, the ISO 1217 reference of 1 bar and 20 °C, or DIN 1343 “normal” conditions of 0 °C. The same airflow can differ by 7% or more between them, so pick the definition on your data sheet under More options. The table shows how many actual cubic feet the same standard flow fills at other line pressures, which is what you need to size pipes, filters and flow meters that see actual volume. This page converts flow between conditions; it is different from the compressed-air leak cost calculator, which prices leaks, and the receiver tank calculator, which sizes storage.
Worked example
A tool needs 100 SCFM (14.7 psia, 68 °F dry air) and is fed at 100 psig with air at 80 °F and 0% humidity at sea level. Atmospheric pressure is 14.70 psia, so the line is at 114.70 psia. The absolute temperatures are 539.67 °R (actual) and 527.67 °R (standard). Actual ÷ standard volume = 14.7 ÷ 114.70 × 539.67 ÷ 527.67 = 0.1311, so ACFM = 100 × 0.1311 = 13.11 ACFM (0.371 m³/min). The dry-air mass flow is 7.52 lb/min (204.6 kg/h). Converting back, 13.11 ACFM at the same conditions is 100 SCFM.
Sources: The Engineering ToolBox: SCFM, ACFM and ICFM (formula with humidity, 5,000 ft example) · Omni Calculator: SCFM calculator (70 °F, 14.7 psia reference; 100 ACFM, 50 °C, 20 psig example)
Frequently asked questions
What is the difference between SCFM and ACFM?
SCFM is the airflow corrected to a reference pressure, temperature and humidity, so it measures the amount of air. ACFM is the volume that air occupies at the real conditions in the system. At 100 psig, 100 SCFM fills only about 13 actual cubic feet per minute because the air is compressed to roughly one-eighth of its free volume.
How do I convert SCFM to ACFM?
Multiply SCFM by the standard absolute pressure divided by the actual absolute pressure, then by the actual absolute temperature divided by the standard absolute temperature. If humidity matters, subtract the water vapour partial pressure from each absolute pressure first. For example, 100 SCFM at 100 psig and 80 °F is 100 × 14.7 ÷ 114.7 × 539.67 ÷ 527.67 = 13.1 ACFM.
Which standard conditions does SCFM use?
It depends on who wrote the number. Common choices are 14.7 psia and 68 °F dry air, 14.696 psia and 60 °F for the US gas industry, 14.7 psia, 68 °F and 36% humidity for ASME standard air, 1 bar (14.5 psia) and 20 °C dry air for ISO 1217, which CAGI compressor data sheets also use, and 1.01325 bar and 0 °C for normal cubic metres (Nm³). Always check the data sheet.
Does altitude change SCFM or ACFM?
Altitude does not change SCFM, which is defined at fixed conditions, but it changes ACFM. At 5,000 ft the atmosphere is about 12.2 psia, so a compressor drawing the same actual volume delivers about 17% less standard air, and air at a given gauge pressure is less dense than at sea level.
What is ICFM or FAD?
Inlet cubic feet per minute (ICFM) and free air delivery (FAD) are actual flows measured at the compressor inlet conditions, that is, ACFM at 0 gauge pressure with the inlet temperature and humidity. Set the line pressure to 0 and enter the inlet temperature, humidity and elevation to convert them to SCFM.
Is SCFM the same as CFM?
Not necessarily. A plain “CFM” on a compressor or tool is usually SCFM or FAD, while CFM on a fan or blower is usually actual volume. If a rating does not state the reference conditions, ask the manufacturer before comparing numbers.
Related calculators
Use these figures for estimating and comparing. For compressor acceptance tests or custody-transfer metering, follow the reference conditions and methods in the applicable standard or contract.