PureAirIndex · Guide
Air changes per hour: the formula, a calculator and real-model numbers
Air changes per hour (ACH) is how many times a room's full volume of air could pass through a purifier's filter in one hour. With a purifier, it is usually called equivalent ACH (eACH), because it measures filtered air, not fresh air.
The formula
ACH = CADR (cfm) × 60 ÷ (floor area ft² × ceiling height ft)
CADR is in cubic feet per minute, so × 60 converts it to cubic feet per hour. Dividing by room volume gives air changes per hour. Rearranged, the CADR you need is: area × ceiling × target ACH ÷ 60.
Worked example
A 300 ft² bedroom with an 8 ft ceiling has 2,400 ft³ of air. A purifier with a smoke CADR of 200 cfm moves 12,000 ft³ per hour. 12,000 ÷ 2,400 = 5 ACH. Raise the ceiling to 10 ft and the same purifier gives 4 ACH.
Quick ACH calculator
How many certified purifiers reach each ACH level?
Count of the 247 ENERGY STAR records whose certified smoke CADR reaches the target, assuming an 8 ft ceiling. This is calculated from our snapshot, not a recommendation.
| Room | 2 ACH | 4 ACH | 4.8 ACH | 6 ACH |
|---|---|---|---|---|
| 150 ft² | 236 (≥40 cfm) | 208 (≥80 cfm) | 200 (≥96 cfm) | 175 (≥120 cfm) |
| 300 ft² | 208 (≥80 cfm) | 125 (≥160 cfm) | 117 (≥192 cfm) | 98 (≥240 cfm) |
| 500 ft² | 167 (≥134 cfm) | 82 (≥267 cfm) | 50 (≥320 cfm) | 19 (≥400 cfm) |
What ACH does not tell you
- Certified CADR is measured at the highest speed. At a quieter setting, real ACH is lower.
- The formula assumes well-mixed air. Furniture, closed doors and placement change results.
- Smoke CADR covers fine particles. It says nothing about gases, odors or health outcomes.
- There is no single "right" ACH. The 4.8 figure used in our calculator is a common sizing comparison, not a medical recommendation.
See the room-coverage study and our methodology.