How this calculator works
Air changes per hour (ACH) say how many times a purifier cleans all of the air in the room each hour. The CADR you need is the volume of the room times the ACH, divided by 60 to turn hours into minutes. The AHAM rule of thumb, that the CADR should be at least two-thirds of the floor area for a room with an 8 ft ceiling, is the same sum at 4.8 air changes an hour.
Worked example: a 15 × 12 ft room (180 ft²) with an 8 ft ceiling is 1,440 ft³. At 4.8 air changes an hour you need 1,440 × 4.8 ÷ 60 = 115 cfm, in line with the 2/3 rule (120 cfm). For a wildfire day at 6 ACH you need 144 cfm.
CADR to look for by room size (8 ft ceilings)
The smoke CADR in cfm, at three levels of cleaning.
| Room | Area | 2 air changes | 4.8 (AHAM 2/3 rule) | 6 air changes |
|---|---|---|---|---|
| 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 |
Tips before you buy
- Use the CADR on the box, not the "covers up to" claim. Coverage numbers are often given at 1 air change an hour, which cleans the air too slowly to help much with allergies or smoke.
- A bigger purifier on a lower speed is quieter than a small one on high. Buy a bit more CADR than you need.
- Choose a true HEPA filter for fine particles and allergens. Activated carbon is for smells and gases, and the amount of carbon matters.
- Run it with the doors and windows closed, put it where air can flow in and out freely, and run it continuously: it does most good when it never switches off.
- Replace the filters on schedule. A clogged filter cuts the airflow, and the CADR on the box is for a clean filter.
- Avoid ozone-producing "air purifiers" and ionizers. Look for models certified as ozone-safe.
Tools & materials for this project
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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.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.