What CADR Actually Measures
Clean Air Delivery Rate comes from a standardized test, ANSI/AHAM AC-1, run by the Association of Home Appliance Manufacturers. A purifier is placed in a sealed 1,008-cubic-foot chamber loaded with a known concentration of particles; the test measures how fast the concentration falls compared with natural settling, and converts that into the equivalent flow of perfectly clean air in cubic feet per minute. Three particle types are tested, and three CADRs are published:
- Smoke (0.09 to 1.0 microns): the smallest, and the closest analog to wildfire smoke and urban PM2.5. Maximum reported value is 450.
- Dust (0.5 to 3 microns): household dust and many allergens. Maximum 400.
- Pollen (5 to 11 microns): the largest, easiest to capture. Maximum 450.
Because smoke particles are the hardest to remove, smoke CADR is always the lowest of the three and is the one to use for sizing. A unit rated 200 smoke / 220 dust / 240 pollen is a 200 CFM purifier for practical purposes. AHAM describes the program and the room-size rule on its air filtration standards page.
CADR folds together two things: filter efficiency and fan airflow. A purifier with a true HEPA filter and a weak fan can post a lower CADR than one with a slightly less efficient filter and a strong fan. That is fine; what you breathe depends on how much clean air arrives, and CADR measures exactly that. It also means CADR is measured on the highest fan speed; run the unit on low and you get a fraction of the rated number.
From CADR to Air Changes per Hour
The more intuitive way to think about cleaning power is air changes per hour (ACH): how many times per hour the purifier processes a volume of air equal to the room. The conversion is simple:
ACH = CADR (CFM) x 60 / room volume (cubic feet)
Room volume is length x width x ceiling height. A 12 x 15 foot bedroom with an 8-foot ceiling is 1,440 cubic feet. A purifier with a smoke CADR of 120 CFM delivers 120 x 60 = 7,200 cubic feet of clean air per hour, which is 5 air changes. Flip it around to find the CADR you need for a target ACH:
Required CADR = target ACH x room volume / 60
The AHAM 2/3 Rule
AHAM's rule of thumb is that a purifier's smoke CADR should be at least two-thirds of the room's floor area in square feet. It is a shortcut for the formula above at an 8-foot ceiling: two-thirds of the area, times 60 minutes, divided by 8 times the area, equals 5 ACH exactly. So “follow the 2/3 rule” and “target about 5 ACH” are the same advice. The rule stops being accurate when ceilings are taller. A 10-foot ceiling adds 25% more volume, so you need 25% more CADR for the same ACH.
How Many Air Changes Do You Need?
| Situation | Target ACH | Why |
|---|---|---|
| General dust, everyday indoor air | About 4 | Keeps background PM2.5 low in a closed room |
| Allergies, asthma, pets | 4 to 5 (the 2/3 rule) | Faster removal of dander, pollen and dust between disturbances |
| Wildfire smoke, heavy outdoor pollution | 6 or more | Particles infiltrate continuously; higher rate keeps indoor levels down |
| Below 2 | Not useful | Cleaning is slower than normal infiltration and resuspension |
These targets are consistent with the EPA's Guide to Air Cleaners in the Home, which recommends choosing a portable air cleaner whose CADR is matched to the room and notes that a higher CADR clears a room faster. The smoke-event number is the one our wildfire smoke purifier guide builds on.
Worked Examples
Small bedroom: 10 x 12 ft, 8-ft ceiling
Floor area 120 sq ft, volume 960 cu ft. The 2/3 rule gives 80 CFM. For allergy relief at 5 ACH: 5 x 960 / 60 = 80 CFM, the same answer. For smoke at 6 ACH: 96 CFM. A compact purifier with a smoke CADR around 100 to 140 comfortably covers this room on medium, which also means it runs quietly.
Living room: 15 x 20 ft, 9-ft ceiling
Floor area 300 sq ft, volume 2,700 cu ft. The 2/3 rule says 200 CFM, but the 9-foot ceiling pushes the real 5-ACH number to 5 x 2,700 / 60 = 225 CFM. For smoke at 6 ACH: 270 CFM. This is mid-to-large purifier territory, and a common mistake is buying a “bedroom” unit rated around 140 CFM and wondering why the living room never clears; it is delivering about 3 ACH, half of what you want.
Open-plan great room: 25 x 30 ft, 10-ft ceiling
Floor area 750 sq ft, volume 7,500 cu ft. At 5 ACH you need 625 CFM, which exceeds the AHAM maximum of 450 for a single unit. Two purifiers at 300+ CFM each, placed at opposite ends, will outperform one giant unit because they also mix the air better. Alternatively, treat the space as separate zones and clean the one you actually occupy.
Get Your Number
You can do the arithmetic above by hand, or let our air purifier size calculator handle it: enter room dimensions and ceiling height, choose your concern (general, allergies, or wildfire smoke), and it returns the target CADR plus purifiers that meet it. Size first, then shop by filter type. Once you have a CADR target, our HEPA vs activated carbon guide helps decide whether you also need a gas-phase stage.
Filtering Fine Particles Indoors
When outdoor particle pollution is elevated it seeps indoors. A True-HEPA purifier captures up to 99.97% of the fine particles — PM2.5, wildfire smoke, dust and pollen — that build up inside, giving sensitive groups cleaner air to breathe.
Compare top-rated True-HEPA purifiers:
Prices are approximate and subject to change. As an Amazon Associate, we earn from qualifying purchases at no extra cost to you.
Reading Spec Sheets Without Getting Fooled
- “Covers up to 1,500 sq ft” often means one air change per hour, which is not meaningful cleaning. Divide the claimed CADR by 2/3 to see the room size the 2/3 rule supports; a 200 CFM unit is honestly a 300 sq ft purifier.
- Missing CADR. If the maker publishes only a filter efficiency and a room size, assume the CADR is low.
- AHAM Verifide mark. Independently tested CADR. Not every good purifier participates, but the mark removes doubt.
- Noise at the speed you will actually use. A unit that hits its CADR only at a loud top speed will be run on low, at a fraction of its rating. Buying one size up and running it on medium is often the quieter, more effective choice.
- Placement matters. Keep intakes clear of walls and furniture, close the door, and expect lower real-world ACH than the chamber test in a leaky room. Checking your city on our air quality rankings tells you how hard the purifier will be working against outdoor infiltration.
Bottom Line
CADR is the spec that predicts results. Use the smoke number, convert it to air changes for your actual room volume, target about 5 ACH for everyday use and 6 or more for smoke, and size up rather than down. Run the calculator, then pick the quietest true HEPA unit that clears your target.
Note: ACH targets are widely used indoor-air guidelines rather than regulatory limits. Real-world performance is lower than chamber ratings in rooms with open doors, leaky windows or heavy ongoing sources.