Air Purifier Sizing Calculator
Enter your room's footprint and ceiling height, pick the use case, and get the minimum CADR (Clean Air Delivery Rate) your purifier needs. Based on the EPA's air-changes-per-hour guidance for residential indoor air.
Size a purifier for your room
Recommended minimum CADR
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What CADR actually measures
Clean Air Delivery Rate is the cubic feet per minute of filtered air a purifier delivers — i.e., the unit's airflow multiplied by its single-pass filter efficiency. AHAM (the Association of Home Appliance Manufacturers) measures it in a sealed 1,008-cubic-foot chamber against three test pollutants:
- Smoke CADR — particles 0.09 to 1.0 µm (the most aggressive test, closest to wildfire and combustion smoke).
- Dust CADR — particles 0.5 to 3 µm.
- Pollen CADR — particles 5 to 11 µm (the easiest to filter; biggest number on the box).
For most home air-quality concerns — wildfire smoke, PM2.5, virus aerosols — use Smoke CADR, not Pollen. Pollen CADR is what marketing leads with; Smoke CADR is what matters.
The math behind this calculator
The formula is straightforward:
Required CADR (CFM) = Room volume (ft³) × ACH ÷ 60
Then we divide by the derating factor to compensate for real-world losses (furniture, leaky doors, partially loaded filters, unit placement in a corner instead of mid-room).
Example: a 14 × 12 × 8 ft bedroom is 1,344 ft³. At 5 ACH (allergies), that's 1,344 × 5 ÷ 60 = 112 CFM of clean air per minute. With a -30% derating, you'd want a purifier with a Smoke CADR of at least 112 ÷ 0.7 ≈ 160 CFM.
Quick-reference sizing table
| Room size (8 ft ceiling) | General use (4 ACH) | Allergies (5 ACH) | Wildfire smoke (6 ACH) |
|---|---|---|---|
| Small (100 ft² / ~9 m²) | ~75 CFM | ~95 CFM | ~115 CFM |
| Medium (200 ft² / ~19 m²) | ~150 CFM | ~190 CFM | ~230 CFM |
| Large bedroom (300 ft² / ~28 m²) | ~230 CFM | ~285 CFM | ~345 CFM |
| Living room (500 ft² / ~46 m²) | ~380 CFM | ~475 CFM | ~570 CFM |
(All values include the recommended -30% derating.)
Notes and caveats
- Buying bigger than the minimum is almost always worth it. A larger purifier run on low is quieter and lasts longer than a smaller one run on high.
- Two smaller units often beat one large one in long rooms, L-shaped open-plans, or homes with two living levels.
- HEPA matters less than CADR. A True HEPA filter is 99.97% efficient at 0.3 µm — but that single-pass number is already baked into the CADR. CADR is what determines how fast your room actually clears.
- A DIY Corsi-Rosenthal box (a box fan + four MERV-13 furnace filters in a cube) delivers ~400 CFM of Smoke CADR for $50-80 in parts. It outperforms most retail units on a dollars-per-CFM basis.
Related guides
Air Purifier Sizing
The longer-form explainer — what CADR measures, why HEPA alone doesn't tell the story, and how to read manufacturer specs critically.
Read guide →DIY Corsi-Rosenthal Box
Step-by-step plans for a box-fan-plus-MERV-13 air cleaner that beats most retail purifiers on cost per CFM. EPA-validated design.
Read guide →Indoor Air Quality
You spend ~90% of your time indoors. Why outdoor AQI doesn't tell the full story, and how to actually clean indoor air without buying snake oil.
Read guide →Wildfire Smoke
What's in wildfire smoke, why AQI can spike hundreds of miles from the fire, and the mitigations that actually work — N95s, MERV-13, box-fan cleaners.
Read guide →Know when to run the purifier with Smog Report
A purifier you forget to turn on doesn't help. Smog Report pushes Live Activities to your lock screen when AQI in your area crosses thresholds you set — so you remember to close windows and run the unit before things get bad.
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