The short answer
For a 12 by 15 ft room (180 sq ft) with an 8 ft ceiling, look for an air purifier with a smoke CADR of at least 120, AHAM's two-thirds rule. For wildfire smoke, AHAM recommends 180, a CADR equal to the floor area.
CADR by room size
Smoke CADR to look for, 8 ft ceiling.
| Room | Sq ft | Everyday | Wildfire smoke |
|---|---|---|---|
| 10×10 ft | 100 | 67 | 100 |
| 10×12 ft | 120 | 80 | 120 |
| 12×15 ft | 180 | 120 | 180 |
| 14×16 ft | 224 | 150 | 224 |
| 16×20 ft | 320 | 214 | 320 |
| 20×25 ft | 500 | 334 | 500 |
How it works
The formula
wildfire CADR = floor area × (ceiling ÷ 8, if higher)
air changes an hour = CADR × 60 ÷ (area × ceiling)
Assumptions
- AHAM's rule of thumb: a smoke CADR of at least two-thirds of the room's area in sq ft, for an 8 ft ceiling (AHAM's own example: a 10 × 12 ft room needs at least 80). For wildfire smoke AHAM recommends a smoke CADR equal to the room's area.
- Ceilings above 8 ft hold more air, so we scale the CADR up in proportion, as AHAM advises a purifier rated for a larger room. Lower ceilings keep the 8 ft figure.
- More than one purifier when the CADR needed is over 600, the top of AHAM's smoke rating range (AC-1-2020); their smoke CADRs added together must meet the total. Metric: 1 CFM = 1.699 m³/h.
Worked example: a 12 × 15 ft living room
180 sq ft, 8 ft ceiling
- Everyday: 180 × 2/3 = 120 smoke CADR
- Wildfire smoke: 180 × 1 = 180
- Air changes: 120 × 60 ÷ (180 × 8) = 5 an hour
Too big or too small
- Too small: it can't keep up. The air never gets clean, and you run it on high, where it's loudest.
- Bigger is fine. An oversized purifier can run on a lower, quieter speed and still meet the CADR.
- Check the label, not the box's room size. Makers' room sizes may assume more air changes or a different ceiling; the AHAM smoke CADR is the number to compare.
- Close the doors: a purifier sized for one room can't clean the whole house.
Questions people ask
What size air purifier do I need?
Look at the smoke CADR on the AHAM label: it should be at least two-thirds of the room's floor area in square feet, for an 8 ft ceiling. A 12 by 15 ft room (180 sq ft) needs a smoke CADR of at least 120.
What CADR do I need for wildfire smoke?
AHAM recommends a smoke CADR equal to the room's floor area for wildfire smoke, so 180 for a 180 sq ft room. That's half again as much as the everyday rule.
What does CADR mean?
Clean Air Delivery Rate: how many cubic feet of clean air a minute the purifier delivers on its highest setting, measured by AHAM for smoke, dust and pollen. Use the smoke number to size it; it's usually the lowest of the three.
What about high ceilings?
The rule assumes an 8 ft ceiling, so scale it up for more air: a 12 by 15 ft room with a 10 ft ceiling needs a smoke CADR of about 150.
How many air changes per hour is that?
Two-thirds of the floor area at an 8 ft ceiling gives 5 air changes an hour; the wildfire rule gives 7.5. Lower fan speeds deliver less than the rated CADR, so size for the speed you'll actually run.