Indoor air · CADR to ACH
How many air changes does your purifier give? CADR to ACH.
Enter the CADR from the box and the room size. The tool gives air changes per hour, the minutes for one full change and whether the unit meets the EPA chart minimum.
Short answer
A purifier with a CADR of 200 CFM in a 15 x 12 ft room with an 8 ft ceiling (1,440 ft3) gives 200 x 60 / 1,440 = 8.3 air changes per hour, one full change every 7.2 minutes. The EPA chart minimum for that 180 sq ft room is 117 CFM, so it clears it.
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How it works, with a worked example
Air changes per hour = CADR in CFM x 60 / room volume in cubic feet. CADR is clean air delivered per minute, so multiplying by 60 gives per hour; dividing by volume says how many room volumes that is. For m3/h input the tool converts at 1 CFM = 1.699 m3/h. This is the same relation as the CFM and ACH calculator, applied to an air cleaner.
Worked example. 15 x 12 x 8 ft = 1,440 ft3. A 200 CFM CADR: 200 x 60 / 1,440 = 8.33 ACH, so 60 / 8.33 = 7.2 minutes for one change. The EPA and ENERGY STAR chart works out to 0.65 CFM per sq ft for an 8 ft ceiling, so the 180 sq ft room needs at least 0.65 x 180 = 117 CFM, which is only about 4.9 ACH. Above 8 ft the tool scales the minimum up by ceiling height divided by 8.
CADR is a rating for particles only. EPA notes the CADR system does not cover gases, and that filtration is greater at higher fan speeds.
Frequently asked questions
How do I convert CADR to air changes per hour?
Multiply the CADR in CFM by 60 and divide by the room volume in cubic feet. For example, 200 CFM in a 1,440 ft3 room is 8.3 ACH.
How many ACH does the EPA chart imply?
The chart gives 0.65 CFM per square foot for an 8 ft ceiling, which is 0.65 x 60 / 8 = about 4.9 air changes per hour.
Can I convert m3/h to CFM?
Yes. One CFM is 1.699 m3/h, so divide m3/h by 1.699 to get CFM.
Does a higher ACH always mean cleaner air?
More air through the filter removes particles faster, but it also depends on the filter, the fan speed, the sources in the room and leakage. ACH is a best-case figure.
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Not professional advice
Not a clinical or safety rating. A purifier does not remove carbon monoxide or radon, and does not replace ventilation or removing the pollution source.