SCFM to CFM Calculator
Understanding SCFM and CFM: The Conversion Explained
Airflow measurement is critical in many engineering and HVAC applications. Two common units used are Standard Cubic Feet per Minute (SCFM) and Actual Cubic Feet per Minute (CFM). While both represent a volume of air moving per unit of time, they differ in their reference conditions.
SCFM (Standard Cubic Feet per Minute): This unit measures airflow volume at a defined set of "standard" conditions, typically:
- Temperature: 70°F (21.1°C)
- Pressure: 1 atmosphere (101.325 kPa or 14.73 psia)
- Relative Humidity: 0% (often assumed dry air)
CFM (Actual Cubic Feet per Minute): This unit measures the actual volume of air moving through a system at its current, real-time operating conditions of temperature and pressure. CFM represents the true volume displaced by a fan or ductwork at the point of measurement.
Why Convert?
Often, equipment is rated in SCFM for standardized comparison. However, for system design, balancing, and diagnosing actual performance, you need to know the CFM at the operating conditions. Converting SCFM to CFM allows engineers and technicians to understand the real-world airflow and ensure systems meet their design specifications.
The Conversion Formula
The conversion from SCFM to CFM relies on the ideal gas law, which relates pressure, volume, and temperature. The formula is derived as follows:
CFM = SCFM * (P_standard / P_actual) * (T_actual / T_standard)
Where:
- CFM is Actual Cubic Feet per Minute.
- SCFM is Standard Cubic Feet per Minute (input).
- P_standard is the standard atmospheric pressure (101.325 kPa).
- P_actual is the actual pressure at operating conditions (input, in kPa).
- T_actual is the actual temperature at operating conditions (input, in Kelvin).
- T_standard is the standard temperature (21.1°C or 294.26 Kelvin).
Important Note: Temperatures must be in an absolute scale (Kelvin) for gas law calculations. To convert Celsius to Kelvin, add 273.15.
How to Use This Calculator
1. Enter the airflow value in SCFM. 2. Input the Temperature in degrees Celsius (°C) at the point of measurement. 3. Input the Pressure in kilopascals (kPa) at the point of measurement. 4. Click the "Convert to CFM" button. The calculator will display the equivalent airflow in Actual Cubic Feet per Minute (CFM).
Example Calculation
Let's say you have a fan rated at 150 SCFM. You are measuring its performance in an environment where the temperature is 30°C and the pressure is 98 kPa.
- SCFM = 150
- Temperature (Actual) = 30°C
- Pressure (Actual) = 98 kPa
Convert temperatures to Kelvin:
- T_actual = 30 + 273.15 = 303.15 K
- T_standard = 21.1 + 273.15 = 294.26 K
CFM = 150 * (101.325 / 98) * (303.15 / 294.26)
CFM = 150 * 1.0339 * 1.0302
CFM ≈ 159.23
So, 150 SCFM under standard conditions would equate to approximately 159.23 CFM at 30°C and 98 kPa. This highlights how air expands at higher temperatures and lower pressures, resulting in a larger actual volume.