How to Calculate Air Changes per Hour

Quick answer

Air changes per hour (ACH) compares the airflow delivered in one hour with the volume of a room. With airflow in cubic feet per minute (CFM), use ACH = CFM × 60 ÷ room volume in cubic feet. In metric units, use ACH = airflow in m³/h ÷ room volume in m³.

Worked example using CFM

1. Room volume = 15 × 12 × 8 = 1,440 ft³.
2. The measured airflow is 240 CFM.
3. Convert airflow to an hourly quantity: 240 × 60 = 14,400 ft³/h.
4. Divide hourly airflow by room volume: 14,400 ÷ 1,440 = 10 ACH.

Suppose a room is 15 ft long, 12 ft wide and 8 ft high:

The Toolnoza Air Changes per Hour Calculator (https://toolnoza.com/tools/air-changes-per-hour-calculator) can do the arithmetic after you measure the room and identify the airflow value.

Metric calculation

For a 40 m³ room and airflow of 400 m³/h, ACH = 400 ÷ 40 = 10. If airflow is in litres per second, multiply by 3.6 to convert L/s to m³/h before dividing by room volume.

Measure internal room dimensions rather than using an outside footprint. If the ceiling height varies, use a reasonable average or calculate separate sections and add their volumes.

What the result does and does not tell you

ACH is a volume-flow comparison. It does not prove that every part of a room receives the same airflow or that all existing air is instantly removed. Air can mix unevenly, and supply/exhaust placement, obstructions, leakage and the actual operating fan setting affect performance.

Use a measured or manufacturer-rated airflow appropriate to the installed system. A fan label may describe a free-air rating that differs from airflow after ducts, bends, filters and pressure losses are included. For ventilation design, infection control, workplace safety or code compliance, use the applicable standard and a qualified professional; this arithmetic is not a design approval.

FAQ

Q: Does 10 ACH mean all the air is replaced ten times? A: It means the stated airflow volume equals ten room volumes per hour. Real air movement is not perfectly uniform, so the figure is not a guarantee that every air particle has been replaced.

Q: Should I use the fan's maximum CFM? A: Use the airflow at the operating condition you are evaluating. Ducts, filters and controls can reduce delivered flow from a nominal maximum.

Further reading

• AIHA room air changes per hour calculator (https://www.aiha.org/ih-calculator-app/calcs/room-air-changes-per-hour.html)
• Calculate ACH with Toolnoza (https://toolnoza.com/tools/air-changes-per-hour-calculator)