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Air purifier size calculator
Air purifiers are rated by CADR — clean air delivery rate, in CFM. Enter your room and how hard you want the purifier to work, and get the minimum CADR to look for.
CADR calculator
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Shop purifiers in this rangeAffiliate link. Look for the smoke CADR on the AHAM Verifide label — it is the hardest of the three ratings to hit.
How CADR sizing works
The Association of Home Appliance Manufacturers (AHAM) recommends a smoke CADR of at least two-thirds of the room's area in square feet, assuming an 8 ft ceiling. That works out to about 5 air changes per hour — the same clean-air target the CDC suggests for homes. The calculator generalises it to any ceiling height:
CADR (CFM) = room area × ceiling height × ACH ÷ 60Quick reference (8 ft ceiling, 5 ACH)
| Room | Area | Minimum smoke CADR |
|---|---|---|
| Small bedroom | 100 sq ft | 67 CFM |
| Bedroom | 200 sq ft | 134 CFM |
| Large bedroom / office | 300 sq ft | 200 CFM |
| Living room | 400 sq ft | 267 CFM |
| Open-plan living area | 600 sq ft | 400 CFM |
Things that make a purifier seem undersized
- Running it on low. CADR is measured on the highest speed. Most people run purifiers quieter, so buy headroom.
- Box-advertised coverage. "Covers up to 1,000 sq ft" often assumes one air change per hour. Check the CADR instead.
- Open doorways. If the room is open to the rest of the house you are cleaning a bigger volume than you measured.
- Placement. Keep it off the wall and away from corners so it can move air freely.
Need to size an air scrubber for a job site instead? Use the air scrubber calculator.
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Every result shows the formula behind it.