Air Changes Per Hour Calculator

About the Air Changes Per Hour Calculator

Calculates the air changes per hour (ACH) a ventilation system delivers from its airflow (CFM) and the room volume.

How to use this calculator

1. Enter the airflow in \(CFM\), which is the air moved per minute.
2. Enter the room volume in cubic feet, \(ft^3\).
3. Use the calculator to compute \(ACH\), the number of air changes per hour.
4. Compare the result with your target ventilation level or room requirement.

The formula explained

The formula \(ACH = \frac{CFM \times 60}{volume}\) calculates how many full room air volumes are moved each hour. It converts minutes to hours by multiplying the airflow by \(60\), then divides by the room volume.

• \(\text{ACH}\) = air changes per hour
• \(\text{CFM}\) = airflow in cubic feet per minute
• \(60\) = minutes per hour
• \(\text{volume}\) = room volume in cubic feet

Step by step method

1. Measure or find the airflow in \(CFM\).
2. Find the room volume in \(ft^3\).
3. Multiply \(CFM\) by \(60\), then divide by \(volume\).

Worked example

Problem. A room has an airflow of \(250\) \(CFM\) and a volume of \(5{,}000\) \(ft^3\). What is the air changes per hour?

1. Start with the formula \(ACH = \frac{CFM \times 60}{volume}\).
2. Substitute the values, \(ACH = \frac{250 \times 60}{5{,}000}\).
3. Compute the result, \(ACH = \frac{15{,}000}{5{,}000} = 3\).

Tips and common mistakes

• Make sure the room volume is in \(ft^3\), not square feet, because ACH uses volume.
• Do not forget the \(60\), since airflow in \(CFM\) is per minute and ACH is per hour.

Frequently asked questions