Tested in real homes
Updated March
To size an air purifier correctly, multiply your room’s square footage by 0.66 to find the minimum Smoke Clean Air Delivery Rate (CADR) you need. This ‘2/3 rule’ guarantees the machine can cycle the air 4.8 times per hour, the standard set by the Association of Home Appliance Manufacturers (AHAM). Ignore the square footage claims printed on the retail box. Brands frequently inflate those numbers by calculating for just one or two air changes per hour, leaving you with a machine that fails to meaningfully reduce ambient dust, pollen, or wildfire smoke. Sizing strictly by Smoke CADR prevents this.
The 2/3 Rule: Your Go-To Formula Using CADR
Always base your math on the Clean Air Delivery Rate (CADR) for smoke, which measures the smallest particles (0.09 to 1.0 microns) and indicates true filtration performance. Multiply your room’s square footage by two-thirds to find your baseline CADR. A 150-square-foot bedroom requires a Smoke CADR of at least 100 to achieve the necessary five air changes per hour. Relying on dust or pollen CADR numbers can artificially inflate the machine’s perceived power, as those larger particles are easier for the fan and filter to capture.
Measure Your Room (No, Really, Grab a Tape Measure)
Grab a tape measure to find the exact length and width of your space, then multiply them to get the total square footage. For open-concept floor plans, do not attempt to size a single purifier for the entire combined kitchen, dining, and living area. Measure only the primary zone where you spend the most time, such as a 20-by-15-foot living room section, and size the unit for that specific 300-square-foot footprint. Standard portable purifiers lack the directional airflow required to pull contaminants from adjacent rooms or hallways.
Oversize by 30% for Peace, Quiet, and Power
CADR ratings are tested on the purifier’s maximum fan speed, which usually produces over 50 decibels of noise—too loud for sleeping or watching television. To maintain clean air on a quieter medium or low setting, purchase a unit with a CADR 30 to 50 percent higher than your 2/3 rule calculation dictates. If your 180-square-foot bedroom requires a 120 CADR, buy a model rated for 180 CADR. This allows the motor to cycle the required air volume at a lower RPM, significantly reducing mechanical whine and airflow noise.
Don’t Forget Ceiling Height: Accounting for Air Volume
Standard CADR calculations assume an 8-foot ceiling. If your room features vaulted or 10-foot ceilings, you must account for the additional cubic air volume. Increase your target CADR proportionally to the height difference. For a 10-foot ceiling—which is 25 percent taller than standard—add 25 percent to your required CADR. A room that normally needs a 200 CADR will require a 250 CADR to maintain the target Air Changes per Hour (ACH). Failing to adjust for vertical space leaves the upper volume of air stagnant and unfiltered.
- Use the ‘Smoke’ CADR rating as your primary number. It represents performance against the smallest, hardest-to-capture particles and is the best all-around metric for sizing.
- In large, open-plan spaces, two smaller purifiers are often more effective and quieter than one enormous unit. This allows you to create multiple zones of clean air.
- Give your purifier breathing room. Place it with at least 12-18 inches of clearance on all sides, away from walls and furniture, to ensure it can pull in and push out air efficiently.
- Before you buy, check the price and availability of replacement filters. An air purifier is useless if its HEPA filter (which needs replacing every 6-12 months) is expensive or constantly out of stock.
Frequently Asked Questions
Conclusion
Measure your room right now and calculate your target Smoke CADR. Once you have that number, check the replacement filter costs for the models you are considering before making your final purchase.
