Air Purifier Size Calculator

AHAM's rule is a smoke CADR of at least two thirds of the room's square footage, for an 8 ft ceiling. A 300 sq ft room needs 200 cfm. For allergies, pets or smoke we size up 25 percent so the purifier can do the job on a quieter speed.

Clean Air Delivery Rate, in cubic feet per minute of filtered air, measured by AHAM for smoke, dust and pollen. Smoke is the hardest test because the particles are smallest, so compare purifiers on the smoke number.

From about 7 W for a desk unit to about 109 W for a large Honeywell on high, measured in AHAM's test. A 55 W unit running 24 hours a day uses about 482 kWh a year, about $83 at the US average rate. Lower speeds draw much less.

From about $28 a year (Winix A230, one $28 filter every 12 months) to about $120 (Levoit Core 600S, a $60 filter about every 6 months). Filter cost often beats electricity, so check it before you buy.

Yes for allergies and smoke, because the air keeps getting recontaminated. Use auto mode or a low speed when the air is clean. The yearly costs in this tool assume high speed all day, so your real bill will be lower.

A HEPA purifier sized to the room reduces fine particles indoors, which is why the EPA recommends portable air cleaners during smoke events. Size by the smoke CADR and keep windows closed.

About 96 cfm of smoke CADR: 144 sq ft x 8 ft = 1,152 cu ft, x 5 air changes an hour / 60 = 96 cfm, which matches AHAM's two-thirds rule. Sized up 25 percent for allergies or pets it is 120 cfm. The Levoit Core 300 (smoke CADR 143) and the Winix A230 (159) both clear it.

At the AHAM minimum of 5 air changes an hour, the purifier moves the room's full volume of air through its filter about once every 12 minutes. In a well-mixed room with no new particles coming in, that cuts a one-time burst of smoke to about 37 percent after 12 minutes and about 14 percent after 24 minutes. A larger purifier or a higher speed shortens those times.

Follow the maker's schedule, which ranges from 6 months (Levoit Core 300, 400S and 600S) to 9 months (Blueair 211i Max) to 12 months (Winix A230 and 5520, Coway AP-1512HH, Airmega 350 and 400). Those intervals assume normal use. Heavy smoke, pets or running on high all day can clog a filter sooner, so replace it early if airflow drops or the filter light comes on.

Yes. AHAM's two-thirds rule assumes an 8 ft ceiling, so this tool sizes by volume instead: square feet x ceiling height x 5 air changes / 60. A 300 sq ft room needs 200 cfm at 8 ft but 250 cfm at 10 ft, 25 percent more.