Air purifier CADR calculator (AHAM room sizing)

Enter your room and we'll compute the clean-air delivery rate (CADR) you need for wildfire smoke — the AHAM two-thirds room rule generalized to any ceiling height and air-change rate — then match three purifier tiers.

Based on the AHAM 2/3 rule extended for smoke (higher ACH). Affiliate links — see disclosure.

What this calculates

required CADR = floor area × ceiling height × air changes per hour ÷ 60. Area in square feet and height in feet give room volume in cubic feet; multiplying by air changes per hour and dividing by 60 minutes converts to cubic feet per minute, which is the unit purifier CADR is quoted in.

The default is 5 air changes per hour, and the selector offers 4 for good general filtration and 6 for heavy smoke. CADR — Clean Air Delivery Rate — is how much genuinely filtered air a machine produces, which is not the same as how much air it moves.

Where the standard comes from

CADR is defined by AHAM (the Association of Home Appliance Manufacturers) and is the number on the certification badge, measured separately for smoke, dust and pollen. Use the smoke figure: it is measured against the smallest particles of the three and is the relevant one here. Ratings assume an 8-foot ceiling.

AHAM's familiar "two-thirds rule" — CADR should be at least ⅔ of the room's floor area — is the same thing as this calculation. Set the ceiling to 8 ft and the rate to 5 air changes and the formula reduces to area × 8 × 5 ÷ 60, which is area × ⅔ exactly. The tool is not proposing a rival standard; it just lets you vary the two assumptions that rule hides.

What this does not tell you

Whether the rated CADR is the CADR you will get. Ratings are measured on the highest fan setting, which is often loud enough that people run the machine lower. A purifier on setting 1 may deliver a third of its badge number, and a machine that is too loud to sleep beside is a machine that gets switched off.

Whether the room is sealed. Air changes assume the volume is bounded. An open door to an unfiltered hallway means you are sizing for a much larger room than you entered.

Filter cost over a smoke season. A cheap machine with expensive proprietary filters can cost more across one bad summer than a dearer one with standard filters. Sizing is only part of the decision — see the purifier guide for the rest.

Anything about gases. CADR measures particles. Wildfire smoke also carries carbon monoxide and volatile organic compounds, which a HEPA filter does not remove; that needs activated carbon, rated separately or not at all.

A worked example

A 12 × 14 ft bedroom is 168 sq ft with 8 ft ceilings. At 5 air changes per hour: 168 × 8 × 5 ÷ 60 = 112 cfm smoke CADR. That is comfortably inside the range of a mid-size consumer unit, and it matches the two-thirds rule (168 × ⅔ = 112).

Raise the ceiling to 10 ft — common in an older house — and the same floor area needs 168 × 10 × 5 ÷ 60 = 140 cfm, a 25% jump the two-thirds rule does not see at all. Push to 6 air changes for heavy smoke and it becomes 168 cfm, half as much again as the rule of thumb suggested. Those two inputs are exactly why this tool exists.

If the machine you want falls short, a Corsi–Rosenthal box adds roughly 300 cfm for about $70 and is usually the cheapest way to close a gap of this size.