Air purifier size calculator

Quick answer

For wildfire smoke, your air purifier needs enough smoke CADR to clean the room at a useful rate. The baseline: look for a smoke CADR of at least two-thirds of the room area at an 8-foot ceiling. Higher ceilings, open layouts, and longer smoke events mean you want more margin. This calculator gives you three tiers—minimum, better, and high-confidence—so you can decide what fits your situation.

How the calculator works

The calculator starts with your room dimensions to find the floor area and volume. It multiplies the area by two-thirds to get a baseline smoke CADR, then adjusts for ceiling height. A room with a 10-foot ceiling needs 25% more CADR than one with an 8-foot ceiling, because there is more air to clean.

The three output tiers are fixed multiples of the baseline:

• Minimum — baseline CADR for the adjusted room size.
• Better — 1.25x the minimum. Adds clearing speed and some margin for open layouts or leakage.
• High-confidence — 1.5x the minimum. Useful for faster clearing, very high ceilings, leaky rooms, or running the purifier on a lower (quieter) fan speed.

The smoke scenario you select shifts the recommendation. Standard indoor air can get by on the minimum. Wildfire smoke season is more aggressive, so the calculator suggests the better target. Vulnerable households or anyone wanting maximum margin should aim for the high-confidence target.

Assumptions

• Baseline assumes a typical 8-foot ceiling. Above that, CADR scales proportionally with volume.
• The two-thirds rule comes from Health Canada portable air cleaner guidance for smoke particles.
• Open-concept layouts, doors that cannot be closed, and leaky windows or doors all reduce effective CADR and may require more capacity or multiple units.
• Severe outdoor smoke conditions may require exceeding even the high-confidence target — especially if your room is open to the rest of the home.
• This calculator estimates one room at a time. If you want to protect multiple rooms, calculate each separately.

What the results mean

Smoke CADR (Clean Air Delivery Rate) measures how fast a purifier removes smoke particles from the air in cubic feet per minute. Higher numbers mean faster cleaning. A purifier that just meets the minimum will eventually clean the air, but a unit that exceeds the target will do it faster and maintain better air quality with less effort.

If you entered an existing purifier's smoke CADR, the calculator compares it against all three tiers so you know whether your current unit is undersized, barely adequate, or comfortably above the recommendation for your scenario.

What not to do

Frequently asked questions

What if my room has a sloped or vaulted ceiling?

Use the average height or the tallest point as a rough guide. The calculator is an estimate, not an engineering spec. Vaulted ceilings hold more air and may need more CADR than the calculator suggests. When in doubt, size up.

Can I oversize an air purifier?

Yes, and it is often a good idea. A larger unit running on a lower fan speed is usually quieter and still cleans the air faster than a smaller unit running at max. Just make sure the unit has an honest smoke CADR rating, not just a marketing room size.

Do I need one purifier per room?

Generally yes. Air purifiers recirculate room air — they do not push cleaned air through walls. If you want protection in multiple rooms, you need a purifier (or a DIY box fan filter) in each room. Close the doors to keep that room's cleaned air contained.

What about portable air conditioners or fans with filters?

Most portable AC units and fans with basic filters do not have a smoke CADR rating and are not designed for fine particle removal. They may move air but do not meaningfully reduce PM2.5. Stick with purpose-built air purifiers that list a smoke CADR.

Why does the calculator use smoke CADR instead of dust CADR?

Wildfire smoke particles are closer in size to tobacco smoke than to household dust. Smoke CADR is the most relevant metric because it is tested with fine particles in the PM2.5 range. Dust and pollen CADR use larger particles and can overstate a purifier's smoke performance.