What Size Air Purifier Do I Need? CADR by Room Size
Part of the project planner: Planning heating, cooling and ventilation.
A 15 × 12 ft room with an 8 ft ceiling needs an air purifier with a CADR of about 115 cfm to change the air 4.8 times an hour, which matches the "2/3 rule" (120 cfm). The chart shows other rooms and levels of cleaning.
CADR to look for by room size
Smoke CADR in cfm, for 8 ft ceilings:
| Room | Area | 2 air changes (light) | 4.8 (AHAM 2/3 rule) | 6 air changes (smoke, pets) |
|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 27 cfm | 64 cfm | 80 cfm |
| 12 × 12 ft | 144 ft² | 38 cfm | 92 cfm | 115 cfm |
| 15 × 12 ft | 180 ft² | 48 cfm | 115 cfm | 144 cfm |
| 16 × 16 ft | 256 ft² | 68 cfm | 164 cfm | 205 cfm |
| 20 × 15 ft | 300 ft² | 80 cfm | 192 cfm | 240 cfm |
| 25 × 20 ft | 500 ft² | 133 cfm | 320 cfm | 400 cfm |
How the ceiling height changes it
For a 15 × 12 ft room at 4.8 air changes an hour:
| Ceiling | Room volume | CADR needed |
|---|---|---|
| 8 ft | 1,440 ft³ | 115 cfm |
| 9 ft | 1,620 ft³ | 130 cfm |
| 10 ft | 1,800 ft³ | 144 cfm |
| 12 ft | 2,160 ft³ | 173 cfm |
Put your own room and a purifier's CADR into the air purifier size calculator to see how many air changes it gives.
Why the box says something different
Many purifiers advertise a room size, such as "covers up to 500 sq ft". That figure is often for one air change an hour, or even for one change in a few hours, which cleans the air far too slowly to help with allergies or smoke. The CADR is measured the same way for every model, so use it. The 2/3 rule (two-thirds of the floor area in cfm) is a quick way to match a CADR to a room, and it works out to about 5 air changes an hour for a room with an 8 ft ceiling.
Choosing a purifier
- True HEPA filter for particles such as pollen, dust, pet dander and smoke. Activated carbon is for smells and gases, and only a thick carbon filter does much.
- Size it for the room it is in, with the door closed. A purifier in the living room does little for a bedroom.
- Look at the noise. A larger purifier on a low speed is quieter than a small one working hard.
- Check the running costs: replacement filters usually cost $30 to $100 a year, and the power use is small.
- Avoid ozone. Choose a model that is certified ozone-free, and avoid "ionizers" that make ozone.
Mistakes that cost people money
- Buying by the "covers up to" claim and not by CADR.
- Putting the purifier in a corner behind furniture, which blocks the airflow.
- Running it for only a few hours a day. It works best running all the time on a low speed.
- Not replacing the filter. A clogged filter delivers much less than the CADR on the box.
Frequently asked questions
What size air purifier do I need for a 300 sq ft room?
A CADR of about 200 cfm for an 8 ft ceiling (2/3 × 300), if you follow the AHAM rule. A purifier rated for 300 sq ft by its maker may have a smaller CADR than this.
What is a good CADR for a bedroom?
For a typical 12 × 12 ft bedroom with an 8 ft ceiling, you need a smoke CADR of about 92 cfm for the standard 4.8 air changes an hour, so look for 100 or more. Go higher for allergies or a pet in the room.
What is CADR?
The Clean Air Delivery Rate, in cubic feet a minute: how much clean air a purifier produces. There are separate figures for smoke, dust and pollen, and the smoke figure is the best guide to fine particles.
How many air changes per hour do I need?
About 4 to 5 is a common target for allergies, and 6 or more during wildfire smoke or for people with asthma. Lower figures give general freshening only.
More guides
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.