The CFM Calculation Formula
**CFM (Cubic Feet per Minute)** is the measure of the volume of air flowing into or out of a space. In HVAC and ventilation design, the most direct method to find the required CFM for ventilation is the **Air Changes per Hour (ACH)** method.
The ACH Method Formula
The total required airflow is calculated by finding the room's volume, multiplying it by the desired ACH, and dividing by 60 to convert the time unit from hours to minutes.
$$\mathbf{CFM = \frac{(Length \times Width \times Height) \times ACH}{60}}$$
Alternative CFM Calculation (Sensible Heat)
When sizing an air conditioner or heat pump, the CFM is often calculated based on the sensible cooling load (BTU/h) and the desired temperature difference ($\Delta T$) between the supply and return air.
$$\mathbf{CFM = \frac{\text{Sensible BTU/h}}{1.08 \times \Delta T}}$$(Where $1.08$ is a constant incorporating the density of air and specific heat.)
Rule of Thumb:
For a quick, rough estimate in residential cooling, HVAC technicians often use the rule: **400 CFM per ton of cooling capacity**. A 3-ton AC unit, for example, typically requires about 1,200 CFM of airflow.