About the Air Changes per Hour Calculator
This air changes per hour (ACH) calculator tells you how many times per hour a fan, ventilation system or air purifier can replace the air in a room. Enter the room’s length, width and ceiling height and the airflow in cubic feet per minute (CFM) or cubic metres per hour, and it returns the ACH, the minutes per air change and how long it takes to remove 99% and 99.9% of an airborne contaminant. Enter a target ACH to see the airflow you would need.
It is useful for homeowners choosing a bathroom fan, range hood or HEPA purifier, HVAC designers checking ventilation rates, workshop and lab users planning fume extraction, and anyone sizing a portable air cleaner by its clean air delivery rate (CADR). For a purifier, use its CADR as the airflow.
Removal times use the well-mixed room model published by the US CDC in its environmental infection control guidelines, which assumes the clean air mixes perfectly with room air and the contaminant source has stopped. Real rooms with poor mixing or ongoing emissions take longer. Required ventilation rates for buildings are set by codes and standards such as ASHRAE 62.1 and 62.2, which this calculator does not apply.
With the default inputs, the air changes per hour is 5 ACH. Change any value above to recalculate instantly.
How to use the air changes per hour calculator
- 1Choose feet and CFM or metres and m³/h.
- 2Enter the room’s length, width and ceiling height.
- 3Enter the airflow of the fan or HVAC supply, or the CADR of an air purifier.
- 4Optionally enter a target ACH to see the airflow you need.
- 5Read the ACH, the minutes per air change and the 99% / 99.9% removal times.
Formula and method
Air changes per hour is the volume of air delivered in an hour divided by the room volume. In imperial units, cubic feet per minute is multiplied by 60 to get cubic feet per hour; in metric, airflow is already in cubic metres per hour. Room volume is length × width × ceiling height.
The removal times come from the well-mixed dilution model in CDC’s Guidelines for Environmental Infection Control (Appendix B): concentration falls exponentially, C = C₀·e^(−ACH·t), so the time to reach 1% of the starting level is ln(100) ÷ ACH hours and to reach 0.1% is ln(1000) ÷ ACH hours. At 6 ACH that is about 46 and 69 minutes, matching CDC’s Table B.1.
- ACH
- Air changes per hour
- CFM
- Airflow in cubic feet per minute
- Q
- Airflow in cubic metres per hour
- V
- Room volume (length × width × height)
- C₀, C
- Starting and remaining contaminant concentration
- t
- Time to reach the chosen removal level
Worked examples
12 × 15 ft bedroom with a 120 CFM purifier
The room holds 15 × 12 × 8 = 1,440 ft³. A 120 CFM purifier moves 7,200 ft³ per hour, which is 7,200 ÷ 1,440 = 5 ACH. Removing 99% takes ln(100) ÷ 5 × 60 ≈ 55 minutes, and 6 ACH would need 6 × 1,440 ÷ 60 = 144 CFM.
5 × 4 m office, 2.7 m ceiling, 300 m³/h supply
The office volume is 5 × 4 × 2.7 = 54 m³. Supplying 300 m³/h gives 300 ÷ 54 ≈ 5.56 ACH, one full air change every 10.8 minutes. Reaching 6 ACH would take 6 × 54 = 324 m³/h.
Bathroom 8 × 5 ft, 8 ft ceiling, 50 CFM fan
The bathroom is 8 × 5 × 8 = 320 ft³. A 50 CFM fan delivers 3,000 ft³/h, or 3,000 ÷ 320 = 9.4 ACH — one change every 6.4 minutes — comfortably above the 8 ACH target, which needs about 43 CFM.
Frequently asked questions
How do you calculate air changes per hour?+
Multiply the airflow in CFM by 60 to get cubic feet per hour, then divide by the room volume in cubic feet. For example, 120 CFM in a 1,440 ft³ room gives 120 × 60 ÷ 1,440 = 5 air changes per hour.
How many air changes per hour do I need?+
It depends on the space and the applicable code. CDC’s ventilation guidance for reducing airborne virus particles suggests aiming for 5 or more air changes per hour of clean (outdoor or filtered) air in occupied rooms, while airborne-infection isolation rooms in hospitals are designed for 12 ACH or more. Building codes and ASHRAE standards set minimum ventilation by floor area and occupancy.
Can I use an air purifier’s CADR to find ACH?+
Yes. CADR (clean air delivery rate) is the filtered airflow in CFM, so enter it as the airflow. A purifier with a 200 CFM smoke CADR in a 2,000 ft³ room delivers 6 clean air changes per hour.
How long does it take to clear the air?+
In a well-mixed room, each air change removes about 63% of what remains. Removing 99% takes about 4.6 air changes and 99.9% about 6.9, so at 6 ACH that is roughly 46 and 69 minutes. Poor mixing makes it slower.
What is the difference between ACH and CFM?+
CFM is an absolute airflow rate, while ACH relates that airflow to room size. The same 100 CFM fan gives 7.5 ACH in a small 800 ft³ room but only 1.5 ACH in a 4,000 ft³ open-plan space.