Air Change to Air Volume Flow (CFM) Calculator

Airflow (CFM or ft3/min) for a volume of space, (m3) can be determined based on the air change per hour, ACH (1/h), which is the number of times the air in a space is being replaced within an hour. In other words, 10 air change means the air enters and exits a room ten times every one hour from the HVAC system in one hour. ACH is sometimes referred to as ACPH (Air Change per Hour).


Generally accepted Air Change per Hour falls between 6 to 12 for general ventilation, with some guidelines requiring a minimum of 15 ACH, such as hospital isolation rooms. The table below shows a generally accepted ACH rates (supply air change) for a list of different types of room. The ACH rates shall be largely dependent on the actual room occupancy and application of areas.

A downloadable Excel File to compute Air Volume Flow, CFM or m3/h, from ACH is available below.

Air Change to Air Volume Flow Calculator (Excel)

The concept of air change can also be equated to airflow, CFM of Supply, Exhaust, or Fresh Air. Supply Air Change relates to the total mixed air volume flow of recirculating and fresh air into the room through an air-conditioning system. Exhaust Air Change relates to the air volume flow of air out of the room through the action of an exhaust fan. Fresh Air Change relates to the air volume flow of fresh air into the room through a fan or the air-conditioning system.

Surprisingly, most split air conditioners do not introduce any fresh air into the room it is cooling. The same air recirculates through the AC system over and over again. Without a proper ventilation, the oxygen level in a concealed 10m2 room of 2.5 m height can only last up to 1.5 days with room occupancy of 5 in moderate level of activities. To estimate the duration that the oxygen level can last for a given space, refer to the calculator below. However, it is worth noting that more often, oxygen depletion is not always the main concern for poorly ventilated enclosed space, but more of CO2 accumulation.


One viable solution to keep fresh air flowing into your room is to install an Exhaust Fan. Exhaust Fans draw away a portion of air from the room, forcing fresh air to infiltrate and displace the exhausted air through windows or door gaps. This way we are able to maintain consistent ACH to the room throughout the night.