Minimum clean air delivery rate (CADR) by EPA’s chart and AHAM’s 2/3 rule, with air changes per hour
Updated October 2026
The clean air delivery rate, in cubic feet per minute, is how much filtered air a portable air cleaner delivers, measured under the AHAM standard and printed on the AHAM Verifide seal as three numbers: tobacco smoke, dust and pollen. Smoke particles are the smallest, so EPA notes the smoke CADR is the most relevant for fine particles; it is also the number the 2/3 rule uses. A higher CADR cleans a room faster.
Measure the room’s length and width in feet and its ceiling height. Optionally enter the smoke CADR of a model you are looking at, and how many you would run in the room. The result gives the minimum CADR by EPA’s chart and by the AHAM 2/3 rule, the air changes per hour at that minimum, and how your unit compares.
EPA’s chart in its Guide to Air Cleaners in the Home works out to 0.65 cfm per square foot at an 8-foot ceiling: 300 sq ft × 0.65 = 195 cfm. AHAM’s rule asks for a smoke CADR of at least two-thirds of the floor area: 168 sq ft × 2/3 = 112 cfm. Both assume 8-foot ceilings, and both organizations say to size up for higher ceilings; the calculator scales by ceiling height ÷ 8. Air changes per hour = CADR × 60 ÷ room volume: 150 cfm × 60 ÷ (168 × 8 = 1,344 cubic feet) = 6.7 an hour.
| Room area | EPA minimum CADR | AHAM 2/3 rule | Air changes per hour at EPA minimum |
|---|---|---|---|
| 100 sq ft | 65 cfm | 67 cfm | 4.9 |
| 200 sq ft | 130 cfm | 134 cfm | 4.9 |
| 300 sq ft | 195 cfm | 200 cfm | 4.9 |
| 400 sq ft | 260 cfm | 267 cfm | 4.9 |
| 500 sq ft | 325 cfm | 334 cfm | 4.9 |
| 600 sq ft | 390 cfm | 400 cfm | 4.9 |
At an 8-foot ceiling the EPA chart works out to just under 5 air changes an hour in every room size, because CADR and volume both scale with floor area.
| Room and purifier | EPA minimum (cfm) | AHAM 2/3 (cfm) | Air changes per hour with the unit(s) |
|---|---|---|---|
| 12 × 14 ft bedroom, 8-ft ceiling, one 150-cfm unit | 109 | 112 | 6.7 |
| 400 sq ft open living room, 10-ft ceiling, one 250-cfm unit | 325 | 334 | 3.8 |
| Same living room with two 250-cfm units | 325 | 334 | 7.5 |
| 600 sq ft basement family room, 8-ft ceiling, one 300-cfm unit | 390 | 400 | 3.8 |
Two smaller units can match one large one: their CADRs add, and spreading them out helps reach the whole room.
EPA advises against air cleaners and furnace filters that intentionally produce ozone, a lung irritant, and notes that no federal agency has approved ozone generators for use in occupied spaces. A portable air cleaner treats one room; it does not replace ventilation or removing the source of a pollutant, and it does little for gases unless it has a sorbent filter rated for them. Run it continuously on a setting near its rated CADR, since lower fan speeds deliver less.
Trusting a "covers up to" square footage that assumes fewer air changes than you want. Using the pollen CADR, usually the highest of the three numbers, for smoke. Ignoring tall or vaulted ceilings. Running the unit on low to keep it quiet, which cuts the delivered CADR. Skipping filter changes, which reduces airflow over time. Treating an open floor plan as one small room.
It does not model gases and odors, whole-house HVAC filters (rated by MERV), air leakage, or how fast a specific room clears after a smoke event. People with asthma or other lung conditions should follow their clinician’s advice on wildfire smoke and indoor air.