What is CADR, and what size air purifier do you need?

Key takeaways

• CADR, the clean air delivery rate, is the volume of filtered air a purifier delivers, in cubic feet per minute, measured under the ANSI/AHAM AC-1 test standard.

• Every certified purifier gets three CADRs, for tobacco smoke, dust, and pollen, which stand in for small, medium, and large particles.

• AHAM's two-thirds rule says to pick a smoke CADR of at least two-thirds of your room's area, so a 200 sq ft room needs about 130 or more.

• The suggested room size on an AHAM Verifide label assumes an 8-foot ceiling and 4.8 air changes per hour.

• For wildfire smoke, AHAM recommends a smoke CADR equal to the room's area in square feet, which raises cleaning to about 7.5 air changes per hour.

What CADR measures

CADR stands for clean air delivery rate. AHAM, the Association of Home Appliance Manufacturers, describes it as the volume of clean air a purifier provides every minute, and it's the main figure for comparing purifiers. The test behind it is ANSI/AHAM AC-1, a standard written and reviewed by a committee of manufacturers, government labs, and academics.

A lab releases particles into a sealed chamber of 1,008 cubic feet, about the size of a 10.5 by 12 foot room with an 8-foot ceiling, and counts how fast they settle on their own. Then it repeats the run with the purifier on. The difference between the two rates, scaled to the chamber's volume, is the CADR.

Per the EPA's guide to air cleaners in the home, the higher the CADR, the more particles a purifier removes and the larger the area it can serve.

Smoke, dust, and pollen ratings

A purifier tested under AC-1 gets three numbers, one for each test particle. Per AHAM's testing FAQ, they cover these size ranges:

The smoke number is the one to size by. AHAM bases its suggested room size on the smoke CADR, and the EPA says to look for a high tobacco smoke CADR if small particles are your concern. The other two still matter for specific needs. AHAM suggests that people in high-pollen areas look for a higher pollen rating, and that anyone bothered by dust pay closer attention to the dust rating.

AHAM's FAQ also notes that the largest CADR the AC-1 chamber can measure is 450, which corresponds to a room of about 698 sq ft.

The two-thirds rule

AHAM's sizing shortcut takes one step: multiply your room's floor area by two-thirds, and choose a purifier with a smoke CADR at least that high. In AHAM's own buying tips, a 200 sq ft room calls for a smoke CADR of at least 132.

To get the floor area, the EPA says to multiply the length and width of the space where the purifier will run. In an open floor plan, count the whole area the unit has to serve.

Where the 4.8 air changes per hour comes from

The two-thirds rule is the reverse of a formula in the AC-1 standard. AHAM's white paper on air changes per hour gives the suggested room size as the smoke CADR multiplied by 1.55. That sizing is built to cut fine smoke particles in the room by 80%.

With an 8-foot ceiling, a purifier in its suggested room size cleans the full volume of air 4.84 times an hour, which AHAM rounds to 4.8. AHAM calls this the minimum equivalent air exchange a purifier needs to keep up with unfiltered air coming into the room, and its label now states that the suggested room size is based on 4.8 air changes per hour.

AHAM's chart shows the payoff of more air changes. At one air change an hour, about 45% of particles are removed; at 4.84, 80%; at 10, 89%. Place a purifier in a room smaller than its rating and the air changes rise above 4.8. Place it in a larger room and they fall below.

You can work out the number for any room yourself: multiply the smoke CADR by 60, then divide by the room's volume in cubic feet (length times width times ceiling height).

Ceiling height

AHAM's suggested room sizes and the EPA's sizing chart both assume an 8-foot ceiling. AHAM's FAQ says that above 8 feet, a purifier's real coverage is smaller than the room size on the box, and the EPA's advice is to choose a unit sized for a larger area.

AHAM's formula handles this directly. Room size equals the smoke CADR times 60, divided by the target air changes times the ceiling height. Rearranged for a 10-foot ceiling at 4.8 air changes, you need a smoke CADR of about 0.8 times your floor area, against about two-thirds at 8 feet.

A sizing table by room size

The everyday column uses the EPA's chart. The 10-foot column applies AHAM's formula at 4.8 air changes per hour, and the wildfire column uses AHAM's wildfire guidance. All figures are smoke CADR in cubic feet per minute.

For the bigger rooms in the right-hand columns, the EPA suggests using more than one purifier when a space is larger than any single product serves. For a 600 sq ft room on smoke days, that could mean two units with a smoke CADR of 300 each, one for each half.

Wildfire smoke

AHAM changed its recommendation for wildfire smoke from a room size of 1.55 times the smoke CADR to a room size equal to the smoke CADR. Per its white paper, that lifts removal in the room from 80% to 86% and the air changes from 4.8 to 7.5 an hour. It also eases the load on the filter by about 35%, which extends its life through a long smoke event.

The EPA's guide to setting up a clean room during a wildfire adds the practical steps: use a purifier that's the right size for the room, close the windows and doors, and run the purifier continuously on its highest fan setting if you can. AHAM notes that long runs in heavy smoke may mean changing the filter sooner, and suggests keeping a spare on hand during wildfire season. Our guide on how to clean the air in your home covers the rest of a clean-room setup.

CARB certification and ENERGY STAR

The California Air Resources Board requires every portable indoor air cleaner sold in California to be certified, and all certified models are tested for electrical safety. Electronic air cleaners also have to meet an ozone limit of 0.050 parts per million. CARB keeps a list of certified air cleaning devices, and the EPA points shoppers to it when choosing a unit that doesn't produce ozone. CARB certification covers safety and ozone; the CADR is where you read cleaning performance.

Efficiency is the other label to check. ENERGY STAR says its certified room air cleaners are more than 50% more energy-efficient than standard models, saving about 211 kWh a year, or roughly $18 to $40 depending on size. For years the program rated efficiency as smoke CADR per watt (CADR/W): clean air delivered for each watt drawn. Its Version 3.0 criteria, required for units made from October 9, 2025, moved to PM2.5 CADR per watt and an integrated energy factor that matches the Department of Energy's test. ENERGY STAR also advises picking a model sized to fit your room, since larger models use more energy.

How to read a purifier label

The AHAM Verifide seal puts the sizing details in one box. The large number is the suggested room size in square feet, based on 4.8 air changes per hour with an 8-foot ceiling. Below it sit the three CADRs for tobacco smoke, dust, and pollen, in cubic feet per minute; size by the smoke figure.

The mark itself tells you who checked the numbers. AHAM says certified models are tested by an independent laboratory and checked through unannounced audits, and a model that doesn't hold its original results gets re-rated.

Running on lower speeds

CADR is measured at a purifier's highest fan speed. AHAM's FAQ explains why: "low" and "medium" mean different things from one brand to another, so the top setting gives every model a fair comparison. On a lower speed, clean air delivered and room coverage drop roughly in step with the fan, and the EPA notes that longer run times increase the amount of air filtered.

A purifier rated above your room's two-thirds number can run on a lower setting overnight and still reach the target, and it has headroom to turn up when someone is cooking or the pollen count is high.

Where to put a purifier

The EPA suggests putting the unit in the room where you spend the most time. Keep it away from curtains and anything else that could block the airflow. If the airflow creates a draft, the EPA suggests turning it away from where you sit or sleep.

A purifier filters particles; it doesn't change humidity. If dry winter air is the issue, our comparison of cool mist and warm mist humidifiers covers that side.

What size purifier should you buy?

Measure the floor area and ceiling, then match the smoke CADR to the room's use.

• A bedroom or office with an 8-foot ceiling: A smoke CADR of at least two-thirds of the floor area. For 150 sq ft, that's 100 or more.

• A room you'll run on low overnight: Size up from the two-thirds number so a lower setting still reaches the target.

• Vaulted or 10-foot ceilings: About 0.8 times the floor area.

• Wildfire country: A smoke CADR equal to the floor area for the room you'll shelter in.

• An open-plan living area: Measure the whole connected space, and use two purifiers if it's larger than one unit serves.

• Pollen or dust as the main concern: Size by smoke CADR, then compare the pollen or dust rating between models.

Frequently asked questions

What is a good CADR for an air purifier?

It depends on the room. AHAM's rule is a smoke CADR of at least two-thirds of the room's area, so about 130 for 200 sq ft and 260 for 400 sq ft.

Which CADR number should I use for sizing?

Smoke. AHAM bases its suggested room size on the smoke CADR, and the EPA points to it for small particles.

Is a higher CADR better?

Yes. A higher CADR removes particles faster and serves a larger area, and ENERGY STAR suggests matching the size to your room to keep energy use in line.

Does ceiling height change what size I need?

Yes. Room sizes assume an 8-foot ceiling, so for a 10-foot ceiling aim for a smoke CADR of about 0.8 times the floor area.

What CADR do I need for wildfire smoke?

One equal to the room's area. AHAM recommends a smoke CADR matching the square footage, such as 300 for a 300 sq ft room.

Can I run an air purifier on low?

Yes. CADR is measured on high, so a unit rated above your room's minimum can run on a lower setting and still keep up.

What does 4.8 air changes per hour mean?

It's the AHAM baseline. A purifier in its suggested room size cleans the room's full air volume 4.8 times an hour, enough to cut fine particles by 80%.