ACH ↔ CFM
Air changes per hour calculator
Work out air changes per hour (ACH) from airflow, the CFM a room needs for a target ACH, or how many air scrubbers or purifiers it takes — in feet or meters.
Not sure what target to use? See recommended air changes per hour by room type.
- Room volume
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- Effective airflow
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- Air changes per hour
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- 99% of particles removed in
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- 99.9% removed in
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The formula
ACH = CFM × 60 ÷ room volume (ft³)And rearranged for sizing equipment:
CFM needed = target ACH × room volume (ft³) ÷ 60In metric, airflow is usually given in m³/h, so there is no ×60: ACH = airflow (m³/h) ÷ room volume (m³). One CFM is 1.699 m³/h.
A 20 × 15 ft room with an 8 ft ceiling holds 2,400 ft³. A purifier moving 300 CFM supplies 300 × 60 = 18,000 ft³ per hour, which is 18,000 ÷ 2,400 = 7.5 air changes per hour. Allow for a 20% loss from filter loading and you are closer to 6 ACH.
How long until the air is clean?
Each air change does not replace all of the air — it dilutes it. In a well-mixed room the airborne concentration falls exponentially, so the time to remove a given fraction depends only on ACH:
| ACH | 99% removed | 99.9% removed |
|---|---|---|
| 2 | 138 min | 207 min |
| 4 | 69 min | 104 min |
| 6 | 46 min | 69 min |
| 8 | 35 min | 52 min |
| 10 | 28 min | 41 min |
| 12 | 23 min | 35 min |
| 15 | 18 min | 28 min |
| 20 | 14 min | 21 min |
| 50 | 6 min | 8 min |
Same model as the CDC's air-change removal table (Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B). Real rooms with furniture, dead corners or poor mixing take longer.
Questions
How do you calculate air changes per hour?
Multiply the airflow in cubic feet per minute (CFM) by 60 to get cubic feet per hour, then divide by the room volume in cubic feet (length × width × height). ACH = CFM × 60 ÷ volume.
How do you convert ACH to CFM?
CFM = ACH × room volume ÷ 60. A 20 × 15 × 8 ft room (2,400 ft³) at 6 ACH needs 6 × 2,400 ÷ 60 = 240 CFM of effective airflow.
How many air changes per hour do I need?
It depends on the space. The CDC suggests aiming for 5 or more air changes per hour of clean air in homes and offices; airborne infection isolation rooms require at least 12; negative-pressure work containments commonly target 4–6 or more. See the full table on our ACH by room type page.
Is the rated CFM the airflow I will actually get?
Usually not. Ratings are typically measured with clean filters and no ducting. Loaded filters, long or bent duct runs and pre-filters reduce real airflow, which is why the calculator lets you apply a 10–30% airflow loss.
How long does it take to clean the air in a room?
In a well-mixed room, removing 99% of airborne particles takes about 4.6 ÷ ACH hours and 99.9% takes about 6.9 ÷ ACH hours. At 6 ACH that is roughly 46 and 69 minutes. Poor mixing makes it slower.
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