The single most important air purifier spec is CADR (Clean Air Delivery Rate, measured in CFM) – it tells you how fast a purifier actually cleans the air in a room of a given size. Buy one with too little CADR and it never keeps up; overpay for too much and you waste money and shelf space. This calculator sizes it for you in one step.
Quick answer: as a rule of thumb, your air purifier’s smoke CADR should be at least two-thirds of your room’s square footage (the AHAM 2/3 rule) for a standard 8-foot ceiling. For allergies, asthma, wildfire smoke, or pets you want more – the tool below adjusts for that automatically using air changes per hour (ACH).
Air Purifier Size Calculator
Enter your room and what you are fighting. We convert it to the minimum CADR (CFM) you should look for, then suggest a matching purifier tier.
| Minimum CADR you need | Room class | Real models that clear it |
|---|---|---|
| Up to 140 CFM | Small (bedroom, office) | Levoit Core 300S (141 CFM, ~$100) |
| 141–240 CFM | Medium (living room) | Levoit Core 400S / Coway AP-1512HH (~$200–230) |
| 241–350 CFM | Large (great room) | Coway Airmega 400 (328–400 CFM) / Levoit Core 600S (~$250–350) |
| Over 350 CFM | Very large / open plan | Airmega 400 + a second unit, or a whole-house approach |
How the math works (so you can trust the number)
Air purifier sizing is not guesswork. The clean-air need for a room is its volume × the air changes per hour (ACH) you want, divided by 60 to get CFM:
Required CADR (CFM) = (room sq ft × ceiling height × ACH) ÷ 60
The EPA and AHAM recommend 4–5 ACH for people with allergies or asthma, and higher (5–6 ACH) during wildfire smoke events. A purifier’s rated room size is usually calculated at a lower ACH, which is why we size by your real goal instead of the box’s headline number. AHAM’s classic shortcut – smoke CADR ≥ 2/3 of the room’s square footage – lands close to a 4.8 ACH target at an 8-foot ceiling.
Writers and bloggers: feel free to link this calculator as a reader resource using the snippet above.
Frequently asked questions
Is a higher CADR always better?
Up to a point. A CADR comfortably above your room’s requirement lets you run the purifier on a quieter, lower speed and still hit your target air changes. But paying for a 400-CFM unit in a 120-square-foot bedroom is wasted money – noise and running cost go up for no real benefit.
What if my space is open-plan?
Size to the total connected square footage, not one room. Once your required CADR passes ~350 CFM, two well-placed medium units often outperform a single large one because they cut down dead spots.
Does ceiling height really matter?
Yes – a 300-square-foot room with a 10-foot vaulted ceiling holds 25% more air than the same room at 8 feet, so it needs proportionally more CADR. That is exactly why this tool asks.
The Home Picker is reader-supported. Product links are affiliate links; we may earn a commission at no cost to you. CADR figures cited are manufacturer/AHAM-verified values.