Enter the airflow in cubic feet per minute (CFM) and select the duct or opening size into the calculator to determine the air speed in miles per hour (MPH).
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CFM to MPH Formula
The following formula is used to calculate the air speed in miles per hour from a flow rate measured in cubic feet per minute.
MPH = CFM / (88 × A)
Variables:
- MPH is the air speed in miles per hour
- CFM is the airflow in cubic feet per minute
- A is the cross-sectional area of the duct or opening in square feet
To calculate MPH from CFM, divide the airflow by 88 times the area in square feet. This works because 1 mile per hour is equal to 88 feet per minute.
What is CFM to MPH?
CFM to MPH is a way of converting volumetric airflow into linear air speed. CFM tells you how much air volume moves each minute, while MPH tells you how fast that air is traveling. Because these are different types of measurements, the conversion requires the cross-sectional area the air is moving through, such as a duct, vent, slot, or opening. A smaller area produces a higher air speed for the same CFM, while a larger area produces a lower air speed.
How to Calculate CFM to MPH?
The following steps outline how to calculate MPH from CFM.
- First, determine the airflow in cubic feet per minute (CFM).
- Next, determine the effective cross-sectional area of the duct or opening in square feet.
- Then calculate the air velocity in feet per minute using: Velocity = CFM / Area.
- Finally, divide the velocity by 88 to convert feet per minute into miles per hour.
CFM to MPH Conversion Table
The table below shows example MPH values for 1000 CFM moving through several common duct and opening sizes.
| Duct / Opening | Area (ft²) | 1000 CFM in MPH |
|---|---|---|
| Small Slot | 0.5 | 22.73 |
| Reference Opening | 1 | 11.36 |
| 1 ft × 2 ft Duct | 2 | 5.68 |
| 2 ft × 2 ft Duct | 4 | 2.84 |
| Large Plenum | 10 | 1.14 |
These examples show that doubling the area cuts the air speed roughly in half when the CFM remains the same.
Example Problem :
Use the following variables as an example problem to test your knowledge.
Flow Rate (CFM) = 1000
Area (A) = 1 ft²
MPH = 1000 / (88 × 1) = 11.36 MPH
This means an airflow of 1000 CFM moving through a 1 square foot opening corresponds to an air speed of approximately 11.36 miles per hour.