Select ACH, airflow or room volume, then enter the two displayed values to calculate the nominal ventilation relationship.
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CFM to ACH Formula
The following formula is used to calculate the air changes per hour from an airflow rate in cubic feet per minute and a room volume in cubic feet.
Variables:
- ACH is the air changes per hour
- CFM is the airflow in cubic feet per minute
- V is the room volume in cubic feet
To calculate the air changes per hour, multiply the airflow in CFM by 60 and divide the result by the room volume in cubic feet.
What is CFM to ACH Conversion?
CFM measures volume flow per minute. ACH is the hourly airflow divided by room volume, a nominal volume-turnover rate. It does not mean every parcel of air is replaced each hour or establish fresh-air delivery, contaminant clearance or compliance by itself. The airflow definition must match the ventilation purpose.
How to Calculate ACH from CFM?
The following steps outline how to calculate air changes per hour from a given airflow rate.
- First, determine the airflow rate in cubic feet per minute (CFM).
- Next, determine the room volume in cubic feet by multiplying length × width × height.
- Then multiply the CFM by 60 to convert the airflow to cubic feet per hour.
- Finally, divide that result by the room volume to find the ACH.
- After inserting the values and calculating the result, check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge.
Airflow (CFM) = 200
Room Volume (V) = 1,000 ft³
ACH = (200 × 60) / 1,000 = 12 air changes per hour
CFM to ACH Conversion Table
The table below lists several sample airflow and room volume combinations along with the resulting air changes per hour.
| Airflow (CFM) | Room Volume (ft³) | ACH |
|---|---|---|
| 80 | 1,000 | 4.8 |
| 100 | 1,000 | 6 |
| 150 | 1,000 | 9 |
| 200 | 1,000 | 12 |
| 250 | 1,500 | 10 |
| 300 | 2,000 | 9 |
| 400 | 2,000 | 12 |
In ventilation and HVAC applications, target ACH values are often specified for different room types. Understanding the CFM to ACH relationship helps compare actual airflow performance to those targets and estimate how much airflow is needed for a desired air exchange rate.
