The Edit — Wellness
AHAM Two-Thirds Rule: Air Purifier CADR to Room Size
AHAM's two-thirds rule is the whole calculation: smoke CADR should be at least 2/3 of the room's floor area, so a 300 sq ft room wants about 200. Listed room sizes are marketing arithmetic.
This guide is research-led: we have not handled these units ourselves. If you buy through our links we may earn a commission — it never changes a pick.Research-led, nothing handled. We may earn from links; it never changes a pick. How we choose
CADR stands for clean air delivery rate, measured in cubic feet per minute under AHAM AC-1, a published test standard. It is the only figure in this category that is independently comparable between brands.
This page reads the AHAM method against 5 purifiers, ranked by rating and then by number of ratings on the method in how we choose. No purifier was handled or tested.
Good fit You want the best-rated unit and can accept a small sample
Not for You want thousands of reports behind the purchase
LEVOIT Vital 200S Air Purifier
Good fit You want a high score with a deep evidence base
Not for You take the listed square footage at face value
Good fit You want the deepest owner record here
Not for You want the largest stated coverage number
SIESEAN Air Purifier, 289 CADR
Good fit You want the tested figure printed rather than inferred
Not for You are buying the 4,200 figure literally
Good fit One machine for a bedroom, run continuously
Not for You expect it to cover a whole floor
What the test actually does
A machine runs in a sealed 1,008 cubic foot chamber that has been seeded with a contaminant. The decay in particle concentration is measured against the natural settling rate, and the difference is the machine’s contribution.
Three CADR figures come out of it, because 3 contaminants are used: tobacco smoke, dust and pollen. Smoke uses the smallest particles and is therefore the hardest test, which makes it the number worth comparing.
Pollen produces the flattering figure. When a listing quotes 1 CADR without saying which, assume the most generous one was chosen.
SIESEAN Air Purifier, 289 CADR
289 CADR printed on the listing, which is the one figure this page argues for — and it converts to a room a third the size of the headline claim.
Current price shown on Amazon. We may earn a commission.
| CADR | 289, stated |
|---|---|
| Listed coverage | 4,200 sq ft, stated |
| At 5 ACH | About 448 sq ft |
289 CADR printed on the listing is the transparency this whole page is arguing for, and it is rare enough to note. Most units in this bracket publish a square footage and leave the tested figure off entirely.
Run that number through the two-thirds rule below and it covers roughly 448 square feet at 5 air changes an hour, against a stated 4,200. Both numbers are defensible; they answer different questions.
Buy it if
- You want the tested figure printed rather than inferred
- A large open-plan space on a low setting
Don't buy it if
- You are buying the 4,200 figure literally
The AHAM two-thirds rule for CADR and room size
AHAM’s guidance is that a machine’s smoke CADR should be at least two-thirds of the room’s floor area in square feet, assuming an 8-foot ceiling. So 300 square feet wants a CADR of about 200.
That rule targets roughly 4.8 air changes per hour, which is the level generally used for allergy and smoke relief. Inverted, it says multiply CADR by about 1.55 to get the area a machine handles properly.
| Smoke CADR | Room by the two-thirds rule |
|---|---|
| 100 | About 155 sq ft |
| 150 | About 233 sq ft |
| 250 | About 388 sq ft |
| 350 | About 543 sq ft |
Compare that table to any listing’s coverage claim. Manufacturers frequently quote area at 1 or 2 air changes per hour, which inflates the figure by 3 to 5 times and describes a machine that technically moves the room’s air rather than one that keeps it clean.
LEVOIT Vital 200S Air Purifier
4.7 across 14,968 ratings, and a listed coverage of 1,875 square feet that the arithmetic below reads very differently.
Current price shown on Amazon. We may earn a commission.
| Listed coverage | 1,875 sq ft, stated |
|---|---|
| Owners praise | Quiet running, pet hair |
| Watch | Coverage assumes low air changes |
4.7 across 14,968 ratings is the strongest combination of score and evidence on this page, and owners praise the quiet running and the handling of pet hair.
The listed 1,875 square feet is the low-air-change figure described above. Treat this as an excellent machine for a bedroom or a living room run continuously, rather than one unit for an open-plan floor.
Buy it if
- You want a high score with a deep evidence base
- Pets are the reason you are buying
Don't buy it if
- You take the listed square footage at face value
What HEPA certifies and what it does not
True HEPA is a defined performance class: at least 99.97% of particles at 0.3 micrometres in a single pass. That size is used because it is the hardest to catch, so performance above and below it is better rather than worse.
HEPA-type and HEPA-like are not the class. They are marketing constructions with no threshold behind them, and a listing using either is telling you the filter did not qualify.
Filtration says nothing about throughput, which is what CADR measures. A superb filter in a weak fan cleans a room slowly, and the pairing is what the tested figure captures.
Activated carbon is a separate stage for gases and odours, and it is not part of the HEPA claim. Carbon also saturates faster than the particle filter, which is a running cost rather than a feature.
WINIX 5510 Air Purifier
32,188 ratings at 4.6 makes this the most-reviewed purifier here, with owners praising the auto mode and odour removal.
Current price shown on Amazon. We may earn a commission.
| Evidence | 32,188 ratings |
|---|---|
| Filtration | True HEPA plus carbon |
| Owners praise | Auto mode, odour removal |
32,188 ratings makes this the most-reviewed purifier on the page, and it pairs True HEPA with a carbon stage.
Owners praise the auto mode and the odour removal specifically, and odour is the carbon stage rather than the HEPA one. Budget for both filters over a year, not just the HEPA.
Buy it if
- You want the deepest owner record here
- Odours are part of the problem
Don't buy it if
- You want the largest stated coverage number
Noise decides whether it runs at all
Every CADR figure is measured at maximum fan speed, which is commonly 50 to 60 decibels. Nobody sleeps next to a machine at maximum speed, so the practical clean air delivery in a bedroom is whatever the unit produces on a setting you can tolerate.
That argues for buying a larger machine than the arithmetic demands and running it low. A unit rated well beyond your room delivers its useful air change quietly, which is the setting it will actually spend its life on.
It is also why the sound figure in decibels is worth as much attention as the coverage claim. A purifier switched off at bedtime has a CADR of zero.
PuroAir 240 HEPA Air Purifier
19,536 ratings on a unit sold for 1,000 square feet, where the useful figure is what it does in a bedroom rather than a floor.
Current price shown on Amazon. We may earn a commission.
| Listed coverage | 1,000 sq ft, stated |
|---|---|
| Filtration | HEPA |
| Owners praise | Allergy relief reported |
19,536 ratings and owners reporting genuine relief from allergy symptoms, which is the outcome people buy this category for.
The 1,000 square foot claim is the same low-air-change convention as everything else here. In a bedroom, run continuously on a low setting, it is doing the job the number was never describing.
Buy it if
- One machine for a bedroom, run continuously
- You want a deep evidence base at a lower price
Don't buy it if
- You expect it to cover a whole floor
Where the highest rating sits
AIRDOCTOR AD3500 Air Purifier
4.8 on 465 ratings, the best score here, on a sealed multi-stage design that stops air leaking around the filter.
Current price shown on Amazon. We may earn a commission.
| Rank here | Highest rated of 5 |
|---|---|
| Filtration | Sealed multi-stage |
| Watch | 465 ratings only |
4.8 across 465 ratings is the best score in this comparison, on a newer listing with few reviews yet.
Sealed multi-stage filtration is the substantive claim, and it addresses a real weakness — a machine that leaks air around the filter delivers less than its filter class suggests. Worth the money if that is the problem you are solving.
Buy it if
- You want the best-rated unit and can accept a small sample
- Filtration quality matters more than price
Don't buy it if
- You want thousands of reports behind the purchase
What to check before buying
Find the smoke CADR, multiply by 1.55, and compare that to the room rather than to the listing’s coverage claim. If no CADR is published at all, treat the square footage as unverifiable.
Then price the filters for a year and check the noise figure on the setting you would sleep through. Winter is when this category is bought, and what actually drops in price in late November covers whether waiting helps. Machines sized by this arithmetic rather than by their box claims are in the air purifier shortlist. A unit that has started smelling is naming its own failed stage, which why an air purifier smells decodes. The same reading of a tested figure against a marketed one runs through robot vacuum suction.
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