2. Your rooms

Start with a room type, then adjust the numbers. Add more rooms to size a whole home.

3. Already have a purifier in mind? Test it

Enter its CADR. If the listing shows three numbers (smoke, dust, pollen), use the smoke CADR, because the two-thirds rule is based on it.

How we calculate this

CADR needed = room volume (floor area x ceiling height) x air changes per hour (ACH) / 60. The ACH targets are 3 for general freshness; 4.8 for allergies, pets, odors and smoke, which is what AHAM’s two-thirds rule works out to; and 6 for asthma or sensitive lungs, a more conservative margin we chose. Indoor smoking adds 1 ACH and heavy outdoor pollution adds 0.5, and a room open to other spaces is treated as 25% larger. Those three adjustments are our own rules of thumb, and the total is capped at 7 ACH. With several goals selected, the highest target is used. The “shop for” figure adds 15% because filters load up and airflow drops over time. The “quieter option” divides by 0.6 so the purifier can run at roughly medium speed. CADR figures come from testing to ANSI/AHAM AC-1. This is planning guidance, not medical advice.

Air purifier size chart

Minimum CADR in cfm by room size, for 8 ft ceilings, before the 15% buffer. For 10 ft ceilings add 25%. The calculator above handles other heights and open layouts. In metric, 10 sq ft is about 1 m2 and 1 cfm is about 1.7 m3/h.

Two worked examples

A 12 x 12 ft bedroom for allergies, 8 ft ceiling. Volume is 1,152 cu ft. At 4.8 ACH that is 1,152 x 4.8 / 60 = 92 cfm, so the minimum CADR is 95. With the 15% buffer, shop for 110 cfm or more.

A 20 x 16 ft living room open to the kitchen, two dogs, wildfire smoke in the area, 8 ft ceiling. The open layout makes the volume 2,560 x 1.25 = 3,200 cu ft. The target is 4.8 + 0.5 = 5.3 ACH, so 3,200 x 5.3 / 60 = 283 cfm. The minimum CADR is 285, and with the buffer you shop for 330 cfm or more.

Why the room size on the box can mislead

Room ratings from independent testing, such as AHAM Verifide and ENERGY STAR, assume about 4.8 air changes per hour. Some makers publish much bigger numbers that assume only one air change per hour. Coway’s own Airmega 400 page, for example, lists 3,120 sq ft at one air change per hour and 1,560 sq ft at two, while the ENERGY STAR room rating for the same model is 583 sq ft. Compare CADR, not coverage claims.

Purifiers with lab-tested CADR

These are the models the calculator draws from. Smoke CADR and rated room size come from the ENERGY STAR database, which lists CADR tested to AHAM AC-1. Model names link to Amazon search results; as an Amazon Associate we earn from qualifying purchases. We have not tested these units ourselves. A carbon filter is marked only where the maker or the database confirms one.

How to measure your room

Use a tape measure and write down three numbers in feet: length, width and ceiling height. Length times width is the floor area. Floor area times ceiling height is the volume of air the purifier has to clean.

For an L-shaped room, split it into two rectangles and add the areas. For a sloped ceiling, use the average height. If the room opens into a hallway or kitchen with no door, tick the “open to another room” box in the calculator, or measure the whole connected space. The EPA gives the same advice for open floor plans.

If you want a second opinion on the result, the EPA publishes a sizing chart in its guide to air cleaners in the home. It rests on the same two-thirds rule, and at the room sizes it lists (100 to 600 sq ft) it differs from the size chart above by 5 cfm at most.

Where to put it and how to run it

Put the purifier in the room where you spend the most time. For most people that is the bedroom, with the door closed overnight. Set it on the floor or a sturdy table with open space around it, and keep curtains and furniture away from the intake and outlet. Manuals usually state the clearance they want, often around a foot, so check yours.

The CADR on the box is measured at the highest fan speed. On a lower setting the unit moves less air, so its real air changes per hour drop with it. That is why the calculator also gives a quieter option: a unit with spare capacity can do the job on medium without the noise of running flat out.

Running it longer and faster filters more air. During a wildfire smoke event, turn it up and keep the windows shut. For everyday dust and pollen, a medium setting running all day tends to do more than a high setting that you switch off because it is loud.

What a purifier cannot do

• It does not remove everything. CADR measures particles only. There is no widely used rating for how well a filter removes gases, and a carbon filter helps mainly when it holds a large amount of carbon.
• It does not fix mold. Mold needs a moisture source removed. A purifier will not solve a damp basement.
• It does not replace source control and fresh air. The EPA lists removing pollution sources and ventilating with clean outdoor air as the most effective steps, with filtration as a supplement.
• It does not protect you from carbon monoxide. Only a carbon monoxide alarm will warn you.

If you want cleaner air through the whole house, the furnace filter matters too. The EPA suggests MERV 13 or the highest rating your system can take, and notes that a furnace filter only works while the system is running, which is usually less than a quarter of the time. An HVAC technician can tell you what your system can handle.

What it costs to run

There are two running costs: electricity and replacement filters.

For electricity, the ENERGY STAR database lists an estimated annual energy use for each certified model. The Coway Airmega 400, for example, is listed at 366 kWh a year. Multiply that by your electricity rate. At 17 cents per kWh, which is only an example, that comes to about $62 a year. ENERGY STAR measures efficiency, not how well a unit cleans, so use it to compare units with similar CADR.

For filters, divide the replacement price by the months the filter lasts and multiply by 12. Makers’ life claims are best cases. Coway says the Airmega 400 filter lasts up to 6 to 12 months, and smoke, pets or heavy use shorten that. Price the filters before you buy the unit. As an illustration with made-up prices, a $100 purifier that needs a $60 filter every 6 months costs about $400 over three years, counting five replacement filters. A $200 purifier with a $60 filter once a year costs about $320, counting two replacements.

Mistakes that lead to the wrong size

• Buying by the “covers X sq ft” claim. Some makers base it on one air change per hour, as explained above.
• Forgetting ceiling height. A 10 ft ceiling needs about 25% more CADR than the same floor area with an 8 ft ceiling.
• Sizing for a closed bedroom and then sleeping with the door open.
• Expecting top-speed performance and quiet at the same time.
• Treating one purifier as a whole-home solution. It cleans the room it is in.
• Skipping filter changes. A clogged filter delivers less air than the CADR on the box.
• Buying a unit that deliberately produces ozone.

Work through the calculator at the top with each room you care about, then compare the models it lists against your budget, noise limit and filter costs.

Air purifier sizing FAQ

Sources

• ENERGY STAR Certified Room Air Cleaners dataset
• US EPA, Guide to Air Cleaners in the Home
• AHAM Verifide, room air cleaner certification program