Air purifier for wildfire smoke
When wildfire smoke settles over your town, the air indoors is your last clean room — and a purifier that’s too small for the space barely helps. The wrong number here isn’t a nuisance; it’s hours of breathing fine particles you thought you were filtering out.
The single spec that decides whether a unit can actually clear your room is its smoke CADR. This page is the sizing method, not a product roundup: enter your room size, and the calculator returns the minimum smoke CADR you should shop for, using the AHAM/EPA 2/3 rule — with a worked-examples table you can read without running the tool.
Smoke-CADR room-sizing calculator
Enter your room’s floor area (length × width, in square feet) and its ceiling height. Click Calculate and you’ll get the minimum smoke CADR to shop for, plus the air changes per hour that minimum delivers in your room. The math is the same 2/3 rule shown in the table below, so you can check it by hand.
Results appear after you press Calculate. Prefer a smoke CADR that is AHAM Verifide — a tested, comparable number rather than a marketing claim.
How to size an air purifier for smoke
CADR stands for Clean Air Delivery Rate — how much clean air, in cubic feet per minute, a purifier delivers for a given pollutant. Purifiers carry separate CADR numbers for smoke, dust, and pollen; for wildfire smoke, the one that matters is the smoke CADR, ideally AHAM Verifide so it’s independently tested.
The sizing shortcut is the 2/3 rule: a portable air cleaner’s smoke CADR should be at least two-thirds of the room’s floor area in square feet (EPA/AHAM). In a formula:
minimum smoke CADR (CFM) ≥ 2/3 × floor area (sq ft)
That guidance assumes a standard 8-foot ceiling. In a room with taller ceilings you have more air volume to clean, so size up — toward a higher CADR, or a second unit. Crucially, a bigger CADR means more air changes per hour: the same room clears faster, which matters most when smoke outdoors is heavy and you’re trying to hold a clean room for hours.
Worked examples: room size → minimum smoke CADR
Common room sizes run through the 2/3 rule. The air-changes-per-hour column assumes an 8-foot ceiling — note it lands near 5 ACH across the board, which is the clearing rate the 2/3 rule is designed to reach.
| Room floor area | Minimum smoke CADR (2/3 rule) | Air changes/hr @ 8-ft ceiling |
|---|---|---|
| 100 sq ft | ≥ 67 CFM | ~5.0 ACH |
| 150 sq ft | ≥ 100 CFM | ~5.0 ACH |
| 250 sq ft | ≥ 167 CFM | ~5.0 ACH |
| 350 sq ft | ≥ 234 CFM | ~5.0 ACH |
| 500 sq ft | ≥ 334 CFM | ~5.0 ACH |
Minimum smoke CADR = 2/3 × floor area (rounded up). ACH = CADR × 60 ÷ (area × 8 ft). Method: EPA / AHAM.
What to look for (beyond the number)
For wildfire smoke you want a HEPA (or true-HEPA- equivalent) particle filter. Wildfire smoke is dominated by fine particles — PM2.5 — and a HEPA filter is built to capture them, which is exactly what the smoke CADR measures. That’s the core of an effective smoke purifier: HEPA-class media, sized by smoke CADR to your room.
An activated-carbon stage can help with smoke odors and some gases. But be honest about its limits: there is no standardized gas-phase CADR, so you can’t compare carbon capacity model-to-model the way you can compare smoke CADR. Treat carbon as a helpful add-on for smell, not a measurable guarantee on gases — and don’t let a carbon claim distract from the particle number that actually clears the smoke you see.
Avoid ozone. The EPA warns against “air purifiers” that work by generating ozone. Ozone is a lung irritant, and these devices don’t effectively clean the air of smoke particles. If a product markets “activated oxygen,” ionization as its main mechanism, or intentional ozone, skip it — especially during a smoke event.
How this compares to a DIY box fan
A store-bought purifier isn’t the only way to hit a target CADR. A Corsi–Rosenthal box — a box fan strapped to MERV-13 furnace filters — can deliver a very high smoke CADR for a fraction of the price, which is why it caught on during smoke season. The same 2/3-rule logic applies: it just needs enough clean-air delivery for your room.
The trade-offs are noise, looks, and the fact that a DIY build isn’t AHAM-tested, so its CADR is estimated rather than verified. If you want the full build and the airflow math, see the Corsi–Rosenthal box guide. Either way, size it to the room, and prefer HEPA-class or MERV-13 media for smoke particles.
Air purifier for smoke FAQ
- What size air purifier do I need for a bedroom?
- Measure the floor area and apply the 2/3 rule. A typical 150 sq ft bedroom needs a smoke CADR of about 100 CFM or higher; a larger 250 sq ft bedroom wants roughly 167 CFM (EPA/AHAM). Run the calculator above for your exact room, and round up rather than down.
- What CADR do I need for a living room?
- Living rooms and open-plan spaces are bigger, so the number climbs. A 350 sq ft room needs a smoke CADR near 234 CFM; a large 500 sq ft great room wants about 334 CFM (2/3 rule). For open floor plans, one very high-CADR unit or two smaller units often works better than a single undersized one.
- HEPA or activated carbon for wildfire smoke?
- Both, with HEPA doing the heavy lifting. HEPA-class media captures the fine PM2.5 particles that define wildfire smoke and is what the smoke CADR measures. Activated carbon helps with odor and some gases, but there’s no standardized gas-phase CADR, so its performance isn’t comparable model-to-model. Prioritize smoke CADR first; treat carbon as a bonus.
- Are ionizers or ozone air purifiers OK for smoke?
- No — the EPA warns against ozone-generating devices. Ozone is a lung irritant and these units don’t effectively remove smoke particles. Choose a filter-based purifier sized by smoke CADR instead, and skip anything whose main mechanism is ozone or “activated oxygen.”
Sources & method
The sizing method — CADR, the smoke CADR, AHAM Verifide certification, the 2/3 rule (smoke CADR at least two-thirds of the room’s floor area in square feet), the value of HEPA particle filtration, the limits of activated carbon, and the warning against ozone-generating purifiers — is relayed from the EPA’s Guide to Air Cleaners in the Home. The calculator applies that rule mechanically: minimum smoke CADR = two-thirds of floor area, and air changes per hour = CADR × 60 ÷ room volume; the worked-examples table uses the identical formula. There is no standardized gas-phase CADR, so we make no numeric claim about gas removal. No spread predictions, no risk editorializing, no evacuation guidance — we relay agency information only, attributed. Reviewed by Marko Visic, MPharm, BSc Physics.