Calculate missing CFM, room volume, or ACH for ventilation by entering any two values in this CFM to ACH converter for HVAC systems.
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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.
ACH = (CFM × 60) / V
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 to ACH conversion is used to determine how many times the air inside a room is replaced in one hour. CFM measures how much air moves each minute, while ACH expresses how often the total room air volume is effectively exchanged over the course of an hour. This relationship is commonly used in ventilation design, HVAC planning, air quality analysis, and building performance calculations.
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.