A coverage area printed on a box is not a lie and it is not a standard. It is a number the manufacturer arrived at by picking an air-change rate and dividing. They almost never say which rate. One division tells you, and it takes ten seconds.
Take the unit’s smoke CADR, multiply by 60, and divide by the claimed square footage times 8. That gives the air changes per hour the claim assumes. EPA’s own sizing table and AHAM’s two-thirds rule both work out to about 5 air changes an hour. Plenty of coverage claims work out to 1. A 200 cfm unit advertised at 1,500 sq ft is exactly that case.
Clean air delivery rate is measured in cubic feet per minute. A room has a volume in cubic feet. Divide the first into the second and you get how long the unit takes to process a roomful of air. EPA defines air changes per hour as the airflow per hour divided by the volume of the space, which written out is:
ACH = CADR × 60 ÷ (floor area × ceiling height)
Both published sizing rules assume an 8-foot ceiling, so to check a manufacturer’s claim on the same basis they use, put 8 in for height.
Run that on the example in the headline. A unit with a smoke CADR of 200 cfm, advertised as covering 1,500 square feet:
200 × 60 = 12,000 cubic feet of air per hour.
1,500 sq ft × 8 ft = 12,000 cubic feet of room.
12,000 ÷ 12,000 = 1.0 air change per hour.
The claim is arithmetically true. Once an hour, that unit moves a volume of air equal to the volume of that room. Whether once an hour is worth buying is a completely different question, and it is the one the box declines to raise.
How much floor area that single machine “covers”, depending only on the air-change rate whoever wrote the box decided to use. 8-foot ceiling throughout.
There are two consumer sizing rules in circulation, and both are published by bodies with no product to sell you.
EPA prints a table in its Guide to Air Cleaners in the Home:
| Room area (sq ft) | 100 | 200 | 300 | 400 | 500 | 600 |
|---|---|---|---|---|---|---|
| Minimum CADR (cfm) | 65 | 130 | 195 | 260 | 325 | 390 |
EPA adds that these figures are calculated on an 8-foot ceiling and that higher ceilings call for a higher CADR. AHAM, the trade association that runs the CADR certification programme, publishes a rule of thumb instead: the CADR should be at least two-thirds of the room’s area, on the same 8-foot basis.
EPA’s table is 65 cfm for 100 sq ft, which is 0.65 cfm per square foot — that division is ours, off their own printed row. AHAM publishes two-thirds, or 0.667. Convert each into air changes in the 8-foot room they both assume:
| Rule | CADR per sq ft | Air changes per hour | Published by |
|---|---|---|---|
| EPA sizing table | 0.65 | 4.9 | EPA, Guide to Air Cleaners in the Home |
| AHAM two-thirds rule | 0.667 | 5.0 | AHAM Verifide |
| Ventilation guidance | — | 5 | EPA and CDC, for indoor spaces, counting filtration |
Three documents written for three different purposes, arriving within a rounding error of each other. That convergence is worth carrying into a shop, because it means 5 air changes an hour is not an enthusiast’s number. It is where the consumer guidance already sits, and it is what a coverage claim is being measured against when it comes out at 1.
Enter a room and it gives the minimum smoke CADR that EPA’s table and AHAM’s rule each call for. Enter the CADR and the coverage area off a box as well and it converts that claim straight into air changes per hour.
Open the calculatorThe same CADR covers wildly different areas depending on the air-change rate chosen. Nobody is falsifying anything; they are choosing the divisor. Here is the same set of units at four different choices, on the standard 8-foot ceiling:
| Smoke CADR | At 5 ACH | At 4 ACH | At 2 ACH | At 1 ACH |
|---|---|---|---|---|
| 100 cfm | 150 sq ft | 188 sq ft | 375 sq ft | 750 sq ft |
| 200 cfm | 300 sq ft | 375 sq ft | 750 sq ft | 1,500 sq ft |
| 300 cfm | 450 sq ft | 563 sq ft | 1,125 sq ft | 2,250 sq ft |
| 400 cfm | 600 sq ft | 750 sq ft | 1,500 sq ft | 3,000 sq ft |
Read across the 200 cfm row and you can watch a 300-square-foot air cleaner turn into a 1,500-square-foot one without anything about the machine changing. Read down the 1,500 sq ft column and you can see what an honest 1,500-square-foot claim would need: 400 cfm to reach even 2 air changes, and about 1,000 cfm to reach 5.
EPA states that the CADR rating "is typically measured at the air cleaner’s highest speed", and separately that an air cleaner will filter less air if it is set at a lower speed. The highest speed is the one most people abandon within a week, because it is the loud one.
AHAM treats this as a sizing problem rather than a noise problem, and the phrasing is worth reading closely: choosing an air cleaner properly sized for the space is, in their words, the best way to ensure the noise level is appropriate. Read the other way round, that is an argument for buying beyond the minimum, so that a speed you can live with still clears the bar.
Neither EPA nor AHAM publishes CADR at each fan speed, and manufacturers generally do not either. So the honest position is that nobody can tell you what your unit delivers on the setting you will actually use. Ask the manufacturer directly before you buy. The answer, or the absence of one, tells you something.
This one is EPA in plain words: there are no widely used performance rating systems for portable air cleaners or filters designed to remove gases, and the CADR rating system is for particles only.
That matters most for the two problems people most often buy for. Wildfire smoke is particles and gases; a unit sized perfectly on CADR is addressing one half of it. Odors, solvents and off-gassing are gases outright, and the CADR figure says nothing about them at all. EPA’s guidance for gases is an activated carbon or other absorbent filter, effective when there is a large amount of material in it — thicker is better — while noting that research has not yet shown plasma, photocatalytic oxidation or UV light to remove gases effectively in portable residential units. So for odors, the number to ask for is the weight of carbon, not the CADR.
And on mold, EPA is blunt. Asked whether portable air cleaners and furnace filters solve mold problems, the guide answers no: they do not address the cause, which is water or moisture in the building. They may catch some particles and help with odor. They will not fix the moisture.
A word on what to expect. EPA’s own summary of the research is deliberately modest: studies using portable HEPA air cleaners have shown improvements in one or more allergy or asthma symptoms, and improvements in cardiovascular and respiratory measures that are, in EPA’s phrasing, typically small and not always noticeable to the individual. We are repeating that here because it is the sentence the marketing leaves out, and because a page that sells you a machine owes you the unglamorous version. If you have a specific health concern, that is a conversation with a clinician, not with a calculator.
Whether a filter is genuinely HEPA or merely described in words that resemble it — EPA’s definition is at least 99.97 % of particles at 0.3 microns, that size being the hardest to catch rather than the smallest. The noise at the speed that produces your sized CADR. How many units an open plan actually needs, which EPA answers by saying to consider the whole space the cleaner serves and to use multiple units where one product will not cover the area. Your HVAC filter, where EPA advises MERV 13 or the highest the system will take, and notes that residential systems usually run less than 25 % of the time during heating and cooling seasons — an in-duct filter does nothing while the system is off. And above all the source: EPA is unambiguous that the most effective approach is removing the sources and ventilating with clean outdoor air, with filtration as a supplement to both.
Every figure above is reproduced from one of these. Nothing here is our own determination about air quality, and the one piece of arithmetic that is ours — dividing EPA’s own table down to a per-square-foot rate — is labelled as ours where it appears.