Enter the airflow in cubic feet per minute (CFM) and select the fan or blower displacement to determine the required revolutions per minute (RPM).
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CFM to RPM Formula
The following formula is used to convert airflow in cubic feet per minute into rotational speed in revolutions per minute when the volume moved per revolution is known.
RPM = CFM / V_{rev}Equivalent relationships include:
- CFM = RPM × Vrev
- RPM = CFM ÷ Vrev
- Vrev = CFM ÷ RPM
Variables:
- RPM is the rotational speed in revolutions per minute
- CFM is the airflow in cubic feet per minute
- Vrev is the volume of air moved per revolution in cubic feet per revolution
To calculate RPM, divide the airflow in CFM by the fan or blower’s effective volume per revolution.
What is CFM to RPM?
CFM to RPM conversion relates an airflow rate to the rotational speed needed to produce that airflow. CFM measures how much air moves through a system each minute, while RPM measures how many times a shaft, fan, or blower turns in that same minute. To convert between them, you must know how much air is moved during one revolution.
If a device moves more air per revolution, fewer RPM are needed to reach a target airflow. If it moves less air per revolution, more RPM are required. This makes the volume per revolution a key value when comparing different fans, blowers, and pumps.
For example, a device moving 1.00 ft³/rev would require 500 RPM to deliver 500 CFM. A device moving only 0.10 ft³/rev would require 5,000 RPM to deliver that same airflow.
How to Calculate CFM to RPM?
The following steps outline how to calculate RPM from CFM.
- First, determine the airflow in cubic feet per minute (CFM).
- Next, determine the volume of air moved per revolution, Vrev, in cubic feet per revolution.
- Then divide the airflow by the volume per revolution using the formula RPM = CFM ÷ Vrev.
- After calculating the result, verify your answer with the calculator above.
Example Problem:
Use the following values as an example problem to test your understanding.
Airflow (CFM) = 500
Volume per Revolution (Vrev) = 0.10 ft³/rev
RPM = 500 ÷ 0.10 = 5,000 RPM
CFM to RPM Conversion Table
The table below shows example RPM values for a 500 CFM airflow using representative fan and blower displacement values.
| Fan / Blower Example | Volume per Revolution (ft³/rev) | 500 CFM in RPM |
|---|---|---|
| Small axial fan | 0.02 | 25,000 RPM |
| Medium axial fan | 0.05 | 10,000 RPM |
| Centrifugal blower | 0.10 | 5,000 RPM |
| Large industrial blower | 0.50 | 1,000 RPM |
| Positive-displacement blower or pump | 1.00 | 500 RPM |
In real systems, the effective volume per revolution can vary with blade shape, system resistance, and operating conditions. Even so, the relationship RPM = CFM ÷ Vrev provides a useful way to estimate speed requirements and compare different devices.