Room Size, Bedroom & Placement

Air purifier size calculator

AHAM publishes the rule in one sentence. This applies it to your room, shows the arithmetic, and names the certified units that clear it.

By Scooter M.

Published September 2, 2026

Last verified September 2, 2026

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The short answer

Multiply your room’s floor area in square feet by two-thirds. That is the minimum smoke CADR to look for, and it is AHAM’s own published rule. A 300 square-foot room needs about 200 CFM at an 8-foot ceiling; higher ceilings need proportionally more.

Calculator

What CADR does your room need?

Room length (ft)

Room width (ft)

Ceiling height (ft) — AHAM assumes 8

There is a pet in this room. Targets 6 air changes an hour instead of AHAM’s 4.8, because the allergen source never stops. Our judgment, applied to AHAM’s method.

Floor area: 180 sq ft
Room volume: 1440 cu ft
AHAM two-thirds minimum: 120 CFM
For 4.8 air changes / hr: 116 CFM
Look for at least: 120 CFM smoke CADR

A machine rated 120 CFM would deliver 5.0 air changes an hour in this room. Buying above this figure lets you run at a lower, quieter fan speed and still clear the threshold.

Certified units that clear 120 CFM

LEVOIT Core 300S — 141 CFM
Blueair Blue Pure 411i Max — 145 CFM
Blueair DustMagnet 5440i — 216 CFM
GermGuardian AC3200 — 225 CFM

The rule, quoted

AHAM states it in a single sentence: the CADR of your air cleaner should be equal to at least two-thirds of the room’s area. And attaches the condition people forget: if your ceilings are higher than 8 feet, an air cleaner rated for a larger room will be necessary.

AHAM’s directory works the same relationship in the other direction to produce the suggested room size printed on the certification seal — room square feet = smoke CADR × 1.55 — and states that the seal figure is based on the air cleaner’s ability to clean and circulate the room’s air 4.8 times per hour. The two forms are the same rule from opposite ends.

Reference table, if you would rather not type

This is the same arithmetic, precomputed at an 8-foot ceiling. It is here in plain HTML deliberately: a calculator that only exists in JavaScript is invisible to search crawlers and to AI assistants, and this table is the part that gets quoted.

Minimum certified smoke CADR by room size, 8 ft ceiling
Room size	AHAM minimum (2/3 rule)	With a pet (6 ACH)	Typical room
100 sq ft	67 CFM	80 CFM	Small bedroom, nursery
150 sq ft	100 CFM	120 CFM	Standard bedroom
200 sq ft	134 CFM	160 CFM	Large bedroom, home office
250 sq ft	167 CFM	200 CFM	Small living room
300 sq ft	200 CFM	240 CFM	Living room
350 sq ft	234 CFM	280 CFM	Large living room
400 sq ft	267 CFM	320 CFM	Open living/dining
500 sq ft	334 CFM	400 CFM	Large open plan
600 sq ft	400 CFM	480 CFM	Very large open plan

The middle column is AHAM’s published rule. The third is our 6 air changes per hour target for pet households, computed from room volume — labeled as ours because it is.

The ceiling-height correction

Every figure above assumes 8 feet, because AHAM does. If your ceiling is higher, the same floor area holds more air and the machine has more work to do. The correction is straightforward: multiply the required CADR by your ceiling height divided by 8.

A 300 sq ft room at different ceiling heights
Ceiling	Volume	Minimum CADR	Effective room size of a 229 CFM unit
8 ft	2,400 cu ft	200 CFM	354 sq ft
9 ft	2,700 cu ft	225 CFM	315 sq ft
10 ft	3,000 cu ft	250 CFM	283 sq ft
12 ft	3,600 cu ft	300 CFM	236 sq ft

The last column uses the Honeywell HPA4300 as the worked example — a machine certified for 354 square feet at AHAM’s assumed 8-foot ceiling covers about 236 square feet in a room with 12-foot ceilings. Same machine, same certified figure, different room.

Certified units and the rooms they clear

Every figure below is the certified suggested room size from AHAM’s directory or the ENERGY STAR database — not the manufacturer’s advertised coverage.

Certified room size, and running cost
Unit	Smoke CADR	Certified room	Electricity / yr
LEVOIT Core Mini	34	53 sq ft	$7.15
Honeywell HPA4100	109	169 sq ft	$17
Blueair 411i Max	145	225 sq ft	$20
LEVOIT Core 400S	231	358 sq ft	$43
Honeywell HPA4300	229	354 sq ft	$34
Winix 5500-2	239	370 sq ft	$65
Winix C710	248	384 sq ft	$43
Honeywell HPA6000	257	398 sq ft	$38
Winix AT41	265	410 sq ft	$50
Blueair 311i Max	283	439 sq ft	$35
Alen BreatheSmart 75i	370	574 sq ft	$56
Coway Airmega 400	376	583 sq ft	$67
LEVOIT Core 600S	391	606 sq ft	$71
Blueair 211i Max	435	674 sq ft	$47

Electricity costs on this page use 18.34¢ per kWh, the U.S. average residential price for June 2026 from the U.S. Energy Information Administration, Electric Power Monthly, Table 5.3. Your rate is on your bill and is probably not this one; multiply the kWh figure by your own rate and you have your number.

Quick picks

One certified pick per size bracket. All figures from the registers.

#	Product	Best for	Certified CADR	Price
1	No photo Up to about 170 sq ft Honeywell InSight HPA4100 109 certified CFM, 16 watts, zero measured ozone.	Bedrooms up to about 170 sq ft	109CFM smoke AHAM VERIFIDE	Check price on Amazon
2	No photo Up to about 350 sq ft Honeywell InSight HPA4300 229 certified CFM on 31 watts — the efficiency pick.	True HEPA on the lowest power draw	229CFM smoke AHAM VERIFIDE	Check price on Amazon
3	Up to about 410 sq ft Winix AT41 265 certified CFM, no ionizer, zero measured ozone.	Pet allergies, whole living room	265CFM smoke AHAM VERIFIDE	Check price on Amazon
4	No photo Up to about 670 sq ft Blueair Blue Pure 211i Max 435 certified CFM and the best air-per-watt figure we found.	Open-plan spaces above 500 sq ft	435CFM smoke AHAM VERIFIDE	Check price on Amazon

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Common questions

What size air purifier do I need?

Multiply your room’s floor area by two-thirds — that is AHAM’s own rule and it gives the minimum smoke CADR to look for, assuming an 8-foot ceiling. A 300 square-foot room needs about 200 CFM. If your ceiling is higher, size up proportionally.

Is the calculator using the manufacturer's coverage figure?

No, and that is the point. It works from certified CADR and AHAM’s published rule. Manufacturer coverage figures are usually quoted at one air change per hour, which produces a number four to five times larger than the certified room size on the same machine’s own spec sheet.

What if my room is an odd shape or open plan?

Use the total floor area of the connected space, not just the part the purifier sits in — air moves freely through an opening. For genuinely large open-plan areas, two correctly sized machines in different zones usually beat one very large one, because air movement across a big space is slow.

Why does ceiling height matter?

Because a purifier cleans a volume, not a floor. AHAM’s suggested room sizes assume 8-foot ceilings; AHAM states that if your ceilings are higher, an air cleaner rated for a larger room is necessary. A 10-foot ceiling in the same footprint is 25% more air to move.

Sources

Figures were read on the dates shown against each source. If one has changed, or if we have got something wrong, tell us and we will correct the page.