Which formula correctly calculates air changes per hour (ACH)?

Which formula correctly calculates air changes per hour (ACH)?

• A. (airflow in CFM) / room volume in ft^3
• B. (airflow in CFM × 60) / room volume in ft^3
• C. (airflow in CFM × room volume in ft^3) / 60
• D. (airflow in CFM × 3600) / room volume in ft^3

Answer: (B)

Air changes per hour tells you how many times the air in the whole room is replaced in one hour. If the ventilation rate is in cubic feet per minute (CFM), you need to convert that flow to an hourly amount by multiplying by 60 minutes in an hour. Then dividing by the room volume (in cubic feet) gives how many room volumes are exchanged per hour, i.e., ACH. So the correct formula is (airflow in CFM × 60) / room volume in ft^3. This yields a dimensionless rate in changes per hour. Using the other forms would misalign the units: without the 60, you’d get changes per minute; mixing in the room volume the wrong way gives nonsensical units; and using 3600 would assume a per-second flow rate, which isn’t the given unit, leading to an incorrect value by a factor of 60.

Air changes per hour tells you how many times the air in the whole room is replaced in one hour. If the ventilation rate is in cubic feet per minute (CFM), you need to convert that flow to an hourly amount by multiplying by 60 minutes in an hour. Then dividing by the room volume (in cubic feet) gives how many room volumes are exchanged per hour, i.e., ACH. So the correct formula is (airflow in CFM × 60) / room volume in ft^3. This yields a dimensionless rate in changes per hour.

Using the other forms would misalign the units: without the 60, you’d get changes per minute; mixing in the room volume the wrong way gives nonsensical units; and using 3600 would assume a per-second flow rate, which isn’t the given unit, leading to an incorrect value by a factor of 60.