Clean Air Delivery Rate is three numbers, not one, and the room size follows from them by arithmetic. AHAM's air filtration standards page defines CADR as the volume of filtered air an air cleaner delivers "with separate scores for tobacco smoke, pollen and dust" and gives the sizing rule: the CADR should be at least two-thirds of the room's area, so a 10-by-12-foot room of 120 square feet wants a smoke CADR of at least 80. The condition that changes the answer is what is in the air and how high the ceiling is. The same page raises the target for wildfire smoke to a smoke CADR equal to the room's area in square feet, and notes that ceilings above 8 feet need a unit rated for a larger room.
CADR is measured under ANSI/AHAM AC-1
The three numbers come from one American National Standard. AHAM's guidance on choosing an air cleaner states that CADR is "a recognized performance testing program where air cleaners are randomly selected and tested at an independent laboratory using the American National Standard, known as ANSI/AHAM-AC-1", and that the AHAM Verifide label carries the CADR ratings and the suggested room size. Three pollutants are scored separately rather than averaged into one figure, because a unit can post a strong pollen number and a weak smoke number. A single unlabeled CADR figure on a product page is therefore not the same statement as the seal. Models carrying the mark can be looked up by certified rating, room size and brand in AHAM's directory of certified air cleaners.
The 2/3 rule and 4.8 air changes an hour are the same sum
AHAM's air changes per hour FAQ explains where the two-thirds rule comes from: certified units are tested to deliver 4.8 air changes per hour in their suggested room size, against a minimum ventilation recommendation of three air changes per hour that indoor air quality researchers use. Work it through on the page's own example. A 120-square-foot room with an 8-foot ceiling holds 960 cubic feet. Changing that volume 4.8 times an hour is 4,608 cubic feet an hour, which is 76.8 cubic feet a minute. The two-thirds rule's answer for that room, from the air filtration standards page, is a smoke CADR of 80 — the same figure with a little headroom. The rule is arithmetic on an 8-foot ceiling, which is why the same page says a taller room needs a bigger rating.
Example: 231 cfm turned into a room size and a yearly bill
Levoit's product page for the Core 400S-P smart air purifier lists a CADR of 231 CFM, rated power of 38 W at AC 120 V 60 Hz, a noise level of 22 to 52 dB, an ideal room size of 1,718 ft² at 1 air change per hour or 358 ft² at 4.8 air changes per hour, a weight of 11 lb, a price of $189.99 against a $219.99 list, and a 2-year limited warranty. The same page lists the 3-stage replacement filter at $49.99. Those are the manufacturer's figures, and the single CADR number is published without naming which of AHAM's three pollutants it describes.
Check the room size against the rule. 231 cubic feet a minute is 13,860 cubic feet an hour; divided by 4.8 air changes that is 2,887 cubic feet; divided by an 8-foot ceiling that is 361 square feet, against the 358 square feet on the manufacturer's page. Run the two-thirds rule instead and 231 × 1.5 gives 346 square feet. The three answers agree within about 4 percent, which is what a sizing claim looks like when the arithmetic behind it is the published one.
Now the running cost. At the 38 watts rated on the product page, running continuously, the unit draws 38 × 8,760 hours = 333 kWh a year, which at 17 cents per kilowatt-hour — the rate assumed here — is $56.59. Add one $49.99 filter a year and the first five years cost 5 × $49.99 = $249.95 in filters alone, against a $189.99 purchase price. The consumable outruns the appliance, which is the same pattern that governs a drinking-water filter and, in reverse, the dehumidifier whose cost sits almost entirely in electricity. This is an example of how the published figures combine, not a recommendation.
Before you buy an air purifier
- Three CADR numbers, for smoke, dust and pollen, rather than one unlabeled figure.
- The smoke CADR against two-thirds of the room's area in square feet, or against the full area where wildfire smoke is the reason for buying.
- The room's ceiling height, since the 4.8-air-changes basis assumes 8 feet.
- The AHAM Verifide listing for the model, looked up by certified CADR and suggested room size in AHAM's own directory.
- The replacement filter price in dollars and the maker's stated replacement interval, multiplied out to a yearly figure.
- The rated power in watts, multiplied by the hours the unit will actually run, at the household's own electricity rate.
- The noise range in dB across fan speeds, because the CADR on the box is the figure at the top speed.