Air Purifier Size Calculator
Required smoke CADR and wildfire rating from room area.
What it does
Purifier shopping reduces to one ratio once marketing falls away. Take the room square feet, multiply by two thirds, and buy that smoke CADR or higher: a 300 ft2 room wants 200 CADR for everyday air and 300 against wildfire smoke. The tool above runs both numbers in either unit system.
The imperial default is a living room at 300 square feet. Two thirds is exactly 200 CADR, matching the standard worked example of a 240 ft2 room at 160 CADR. Wildfire guidance drops the discount entirely, so the same room wants 300 CADR when skies turn orange.
The rule comes from the appliance standards world. AHAM guidance sets the two thirds minimum for smoke CADR with room size parity for wildfire smoke, and its Verifide program certifies ratings through independent labs at about 4.8 air changes an hour. EPA calls CADR the most helpful portable cleaner metric.
CADR beats every other spec for comparison, but it is not the whole purchase. True HEPA filtration, filter replacement cost, noise at bedtime speeds, and closed doors between rooms all shape real results. One unit cannot equalize a whole floor no matter what its sticker claims.
How to use
- Pick imperial (ft2) or metric (m2).
- Type the room floor area.
- Read smoke CADR first for everyday use.
- Read the wildfire line during smoke season.
- Shop certified ratings at or above the figure.
Formula
- area
- Room floor area.
- CADR
- Clean air delivery rate for smoke.
- wildfire
- Full area parity for smoke events.
AHAM two thirds rule for everyday air; full parity for wildfire smoke. Tall ceilings and open plans need upsizing beyond the ratio.
Worked example
Example: 300 ft2 room
Imperial default: 300 ft2.
- Everyday: 300 x 2 / 3 = 200 CADR.
- Wildfire: 300 x 1 = 300 CADR smoke.
- Sense check: 240 ft2 gives 160, matching guidance.
- Shop: certified 200+ smoke CADR, True HEPA.
- Tool shows 200 CADR, 300 CADR smoke, 300 ft2.
Reference table
| Room | Everyday | Wildfire |
|---|---|---|
| 120 ft2 | 80 CADR | 120 CADR |
| 150 ft2 | 100 CADR | 150 CADR |
| 300 ft2 | 200 CADR | 300 CADR |
| 450 ft2 | 300 CADR | 450 CADR |
Limits
- One room only; doors and floors break coverage.
- 8 ft ceilings assumed; taller needs more.
- Smoke CADR quoted; dust and pollen differ.
- Rule sizes air cleaning, not heating or humidity.
Common mistakes
- Shopping watts instead of CADR.
- Sizing one unit for a whole floor.
- Ignoring wildfire parity in smoke season.
- Trusting HEPA type as certified True HEPA.
- Forgetting filter replacement budgets.
Try it yourself
Quick checks. Answers sit tucked under each question.
150 ft2 room?
150 x 2/3 = 100 CADR, 150 for wildfire.
450 ft2 open plan?
300 CADR everyday, 450 wildfire, or split two units.
28 m2 room?
28 m2 is 301.4 ft2, so 201 CADR everyday.
Questions
What CADR for 300 ft2?
200 everyday, 300 for wildfire smoke.
What is CADR?
Certified clean air delivery rate for smoke, dust, pollen.
Why two numbers?
Everyday uses two thirds; wildfire wants full parity.
Bigger always better?
To a point; oversize adds noise and energy.
One unit per floor?
No. Closed rooms each need their own sizing.
True HEPA matter?
Yes. Type and like labels skip certification.