Air changes per hour (ACH)
ACH = (CADR × 60) ÷ room volume. Allergy and asthma guidance generally targets 4–5 air changes per hour; 2 per hour is a reasonable minimum for general air cleaning.
"Air changes per hour" (ACH) is how many times a purifier cleans all the air in a room each hour. It's the clearest way to check whether a specific purifier is genuinely strong enough for your space. Enter your room size, ceiling height, and the purifier's CADR above to see the number.
Key takeaways
- 2 air changes per hour is a sensible minimum for general air cleaning.
- 4–5 per hour is the target for allergies and asthma — enough turnover to keep particle levels consistently low.
- Ceiling height matters: a tall room needs more airflow for the same result.
Reading your result
If the calculator returns four or five air changes per hour, that purifier is well matched to your room. Two to four is fine for general use but light for serious allergy relief — you'd run it on a higher speed or size up. Below two, the unit is underpowered for the space.
Sizing from scratch
If you're still shopping and don't have a specific model in mind, start with the room-size to CADR calculator to get a target, then come back here to sanity-check a shortlist. For the background on these numbers, see what CADR and CFM mean.
Frequently asked questions
How many air changes per hour do I need?
For general air cleaning, two air changes per hour is a reasonable minimum. For allergies and asthma, aim for four to five per hour, which is the level most guidance recommends. The calculator above tells you what a given purifier delivers in your room.
What's the formula for air changes per hour?
Air changes per hour (ACH) = (CADR in CFM × 60) ÷ room volume in cubic feet. Room volume is your floor area times the ceiling height. The calculator does this for you.
Does a higher ceiling reduce air changes?
Yes. A taller ceiling means more air volume to clean, so the same purifier delivers fewer air changes per hour. The calculator lets you set your ceiling height so the result is accurate for your room.
Embed this calculator (free)
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