When sizing an air purifier for large room layouts, start with one number: the Clean Air Delivery Rate (CADR). The Association of Home Appliance Manufacturers (AHAM) recommends a CADR equal to at least two thirds of the room’s floor area in square feet, assuming a standard 8 foot ceiling. A 600 square foot great room therefore needs a CADR of about 400 or more. Taller ceilings and wildfire smoke push that number higher.
What size air purifier do I need? The two-thirds rule
CADR is a lab-tested rating, measured in cubic feet per minute, that tells you how much filtered air a unit delivers. AHAM publishes separate CADR figures for smoke, dust and pollen, and a higher score means the unit cleans the air faster. According to AHAM’s air filtration standards page, the CADR of your air cleaner should equal at least two thirds of the room’s area. For wildfire smoke, AHAM suggests going further: a smoke CADR equal to the full square footage of the room.
- Measure the room’s length and width in feet and multiply them to get square feet.
- Multiply the square footage by 0.67 (two thirds). That is your minimum CADR for an 8 foot ceiling.
- If the ceiling is taller than 8 feet, scale up: multiply the result by your ceiling height divided by 8.
- Use the smoke CADR when comparing units, since smoke particles are the smallest and hardest of the three tested pollutants.
The EPA’s Guide to Air Cleaners in the Home uses the same approach, with a chart that runs from a minimum CADR of 65 for a 100 square foot room up to 390 for a 600 square foot room, based on an 8 foot ceiling.
CADR for room size: worked examples
The table below applies the two-thirds rule to common large rooms. The last column adjusts for a 10 foot ceiling.
| Room size (sq ft) | Example space | Minimum CADR, 8 ft ceiling | Minimum CADR, 10 ft ceiling |
|---|---|---|---|
| 300 | Large primary bedroom | 200 | 250 |
| 400 | Family room | 267 | 333 |
| 500 | Living room with dining nook | 333 | 417 |
| 600 | Great room | 400 | 500 |
| 800 | Open kitchen and living area | 533 | 667 |
| 1,000 | Open plan main floor | 667 | 833 |
| 1,200 | Basement rec room or studio | 800 | 1,000 |
For very large open spaces, two mid-size units at opposite ends often work better than one unit in a corner.
Air changes per hour and why the rule works
Air changes per hour (ACH) describes how many times the full volume of air in a room passes through the purifier each hour. AHAM’s white paper on portable air cleaners and air changes per hour gives the formula: ACH equals smoke CADR times 60, divided by the room volume in cubic feet. The same paper states that an air cleaner used in its suggested room size delivers about 4.8 air changes per hour.
Example: a 20 by 25 foot room with an 8 foot ceiling holds 4,000 cubic feet. A smoke CADR of 333 gives about 5 air changes per hour. With a 12 foot ceiling the same unit drops to about 3.3.
HEPA vs other filters
The EPA explains on its What is a HEPA filter? page that a HEPA filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria and other airborne particles at 0.3 microns, the particle size that is hardest to catch.
Other technologies you will see on spec sheets:
- Pre-filters catch hair and large dust so the HEPA stage lasts longer.
- Carbon or deodorization filters target odors and some gases, which CADR does not measure.
- Ionizers make particles settle onto surfaces rather than trapping them.
- UV-C lamps treat microorganisms in the airflow but are not a particle filter.
A caution about ozone
The EPA home guide advises avoiding units that intentionally produce ozone, a lung irritant, and notes that ionizers, electrostatic precipitators, plasma cleaners and UV lights without adequate lamp coatings may emit ozone. It suggests checking California Air Resources Board or AHAM lists of low-ozone units. If a purifier has an ionizer or UV feature, ask the manufacturer whether it has been tested for ozone and whether the feature can be switched off.
Where to place an air purifier
The EPA recommends putting the unit where you spend the most time and keeping it clear of curtains and anything else that blocks airflow. In practice:
- In an open plan room, place the unit near the seating or sleeping area rather than in a far corner.
- In a basement or bedroom that also has a heater, keep the purifier away from the heat outlet so neither unit is fighting the other’s airflow.
Planning a cozier basement or bedroom? Our guides to an electric fireplace in a basement and an electric fireplace in a bedroom cover heat output and placement for the same spaces.
Noise and filter cost: the ownership factors
Check the decibel rating at the fan speed you will actually use, not just the sleep setting. The EPA uses about 50 decibels, roughly a modern refrigerator, as a reference. Sizing up so the unit runs at a lower speed is often the quietest choice.
Filter replacement is the main running cost. The EPA says to replace filters as the manufacturer recommends, typically every 60 to 90 days, and sooner if they look heavily soiled. Before buying, check the price and availability of replacement filters for every stage.
Air purifier for large room use: models we carry
Many of our listings do not publish a CADR, and we will not guess one. If a model you like has no CADR on the page, call us at 866-777-2590 (Mon to Fri, 9AM to 6PM PST) and we will check with the manufacturer.
- Ruhens WonBong 4 Filtration (Egg WHA200): the listing describes 4-stage filtration (pre-filter, deodorization, antibacterial and HEPA), an air quality sensor, a sleep mode, a timer and a cluster ionizer. Because it includes an ionizer, see the ozone note above. No CADR or room coverage is listed.
- Ruhens Air Purifier Egg WHA-200: this listing names a HEPA filter but gives no CADR or coverage figure.
- Ruhens WonBong 3 Filtration 360 (Healing WHA320): offered in one listing and a second listing. Both describe 3-stage, 360 degree filtration without a CADR.
- Bluezone Viral Kill units: the 10-BZ-9009-RTGW (white), 10-BZ-9009-RTGB (black) and 10-BZ-450VK are commercial UV-C units aimed at restaurants and similar spaces. The listings state up to 9,000 cubic feet of treated space, 350 watts and 120 volt power. That treated space figure is not a CADR, and no HEPA particle filter is described, so they are not a direct substitute for a HEPA purifier in a home.
Browse the full Kahuna lineup for Ruhens models. Orders of $999 or more ship free by freight, with a $200 flat rate below that, and returns follow our return and refund policy. For more sizing guides, visit our buying guides hub.
A note on health expectations
Air purifiers reduce some airborne particles. They do not replace ventilation or removing the source of a problem. The EPA notes that studies of portable HEPA units show improvement in some allergy or asthma symptoms, but not always major improvement. If you have asthma or allergies, talk to your doctor about what will help most.
Frequently asked questions
What size air purifier do I need for a 500 square foot room?
Look for a smoke CADR of at least 333 with an 8 foot ceiling, or about 417 with a 10 foot ceiling. If wildfire smoke is a concern, AHAM suggests a smoke CADR of 500.
How many air changes per hour should an air purifier provide?
AHAM’s white paper states that a unit used in its suggested room size delivers about 4.8 air changes per hour. Calculate yours as smoke CADR times 60, divided by room volume in cubic feet.
Is HEPA better than an ionizer?
A HEPA filter traps particles, while an ionizer makes them settle onto surfaces, and the EPA notes ionizers may emit ozone. For a living space, a HEPA-based unit is usually the simpler choice.
What if the listing does not show a CADR?
Call us at 866-777-2590 and we will ask the manufacturer, or search the model in AHAM’s certified directory before you buy.
