What CADR Do I Need for My Room?
Apply AHAM's 2/3 rule: multiply your room's square footage by 0.67 to get the minimum Smoke CADR (in CFM) you need. A 300 sq ft room wants a Smoke CADR of at least ~200. Use the Smoke number specifically — it's the smallest, hardest-to-capture particle class, not the more flattering Dust or Pollen figures also printed on the box.
The one thing to know: CADR (Clean Air Delivery Rate) is an AHAM-administered test measured in CFM (cubic feet per minute) for three separate particle types — smoke, dust, and pollen — because smaller particles are harder to capture. AHAM's sizing guidance (the '2/3 rule') says the unit's Smoke CADR should be at least two-thirds of the room's square footage, e.g. a 300 sq ft room wants a Smoke CADR of at least ~200. Always size by the Smoke number, the smallest and hardest-to-catch particle class — a high Pollen CADR on the box can mask a mediocre Smoke CADR.
Worked examples
| Room size | Minimum Smoke CADR (2/3 rule) | Comfortable sizing (with headroom) |
|---|---|---|
| 150 sq ft (bedroom) | ~100 CFM | ~150 CFM |
| 250 sq ft (large bedroom/office) | ~168 CFM | ~220-250 CFM |
| 400 sq ft (living room) | ~268 CFM | ~350-400 CFM |
| 700 sq ft (open-plan) | ~469 CFM | ~550-650 CFM |
Why sizing up matters beyond "just in case"
An oversized unit for its room isn't wasted capacity — it's a noise-management strategy. Higher CADR generally requires a higher fan speed, and higher fan speed means more noise (see Sleep Mode dB Truth). A purifier sized with headroom can run at a low, quiet speed continuously and still deliver the air-change rate you actually need, where a barely-adequate unit has to run near its loud top speed to keep up.
Square footage vs. real ACH
The 2/3 rule is a square-footage shortcut that assumes a standard ~8 ft ceiling. For rooms with notably higher or lower ceilings, the more precise method is the ACH formula — CADR × 60 ÷ actual room volume in cubic feet — which accounts for the true air volume you're filtering. See the full math in The Air Purifier Numbers.
This page is general sizing guidance based on published AHAM standards, not a safety determination for your specific room or air-quality concern.
Frequently Asked Questions
Applying the AHAM 2/3 rule (150 × 0.67), you want a Smoke CADR of at least ~100 CFM. Sizing up to ~150 CFM gives headroom to run the unit on a quieter, lower fan speed at night while still hitting the target.
400 × 0.67 = a Smoke CADR of at least ~268 CFM. Most 'large room' purifiers in the 300-400+ CFM Smoke CADR range comfortably cover this size with room to run quietly.
Square footage via the 2/3 rule is the quick AHAM shortcut; ACH (air changes per hour) is the more precise math because it accounts for ceiling height. A 300 sq ft room with 8 ft ceilings and a 300 sq ft room with 12 ft ceilings need different CADR for the same air-change rate — the 2/3 rule alone misses that difference.
Smoke CADR. It measures the smallest, hardest-to-capture particle class (0.1-1.0 microns), making it the most conservative and realistic sizing basis. The Pollen CADR is usually the highest number on the box and the easiest one for marketing to lead with.
It's a reasonable strategy for noise management — an oversized unit can run on a low, quiet fan speed and still hit your ACH target, versus an adequately-sized unit running loud on high. But it doesn't fix a wrong filter type: an oversized unit with a 'HEPA-type' (not True HEPA) filter is still an unverified filtration claim.