Garden Air purifier size calculator
Manufacturers quote a "room size" that usually assumes a single air change per hour, which is far too little for allergies or smoke. The number to compare is CADR — clean air delivery rate, in cubic feet per minute — and the industry two-thirds rule ties it directly to your floor area.
How this is calculated
AHAM's two-thirds rule: minimum CADR (cfm) = floor area (sq ft) × ⅔, for an 8 ft ceiling. For a target air change rate: CADR = room volume × ACH ÷ 60.
Source: AHAM AC-1 two-thirds rule, as used in the AHAM Verifide air cleaner program and cited by the US EPA
Worth knowing: The two-thirds rule assumes an 8 ft ceiling and delivers roughly 5 air changes per hour on paper — but CADR is measured on the highest fan speed, which on many units is far too loud to run continuously. Check the CADR at a speed you would actually live with. A unit rated for a bigger room, run slow and quiet, usually beats a small one run flat out.
Common questions
What is CADR?
Clean air delivery rate — the volume of filtered air a purifier delivers each minute, in cubic feet. It is measured independently under the AHAM AC-1 standard, separately for smoke, dust and pollen. Unlike "recommended room size", it is directly comparable between brands.
How many air changes per hour do I need?
Two is adequate for general freshening. Four to five is the usual recommendation for allergies, asthma or smoke. During wildfire smoke events, higher is better — which is why people often size well above the box recommendation.
Is the room size on the box reliable?
Treat it with caution. It commonly assumes a single air change per hour, which is far below what anyone buying a purifier for allergies actually wants. Work from CADR and your own target air change rate instead.
Affiliate Disclosure
Some links on this page are affiliate links. As an Amazon Associate, we earn from qualifying purchases — at no extra cost to you. We do not buy, test, or physically handle the products we cover; our rankings come from aggregated owner reviews, ratings and expert consensus. Commissions never change our verdicts. How we rank · full disclosure.