What size air purifier for your room: the math, worked out

The rule for air purifier room size is short: choose a unit whose smoke CADR is at least two-thirds of the room’s floor area in square feet. A 12 by 15 foot room is 180 square feet, and two-thirds of 180 is 120, so look for a smoke CADR of at least 120. Both EPA and AHAM publish this rule, and it assumes an 8-foot ceiling; EPA adds that a unit does best with doors and windows closed. It is a starting point for ordinary use, and wildfire smoke, tall ceilings and open-plan rooms each change the math in ways set out below. CADR, the clean air delivery rate, is the clean-air number on the label; if it is new to you, what a CADR rating measures explains it first.

The calculation in three steps

1. Measure the floor. Multiply length by width in feet. For an L-shaped room, split it into two rectangles and add them.
2. Take two-thirds. Multiply the area by 2, then divide by 3. Round up.
3. Compare with the smoke CADR. Use the smoke number on the label, not the dust or pollen number. EPA says tobacco smoke CADRs “should be used as the most conservative estimate” for small particles such as PM2.5.

AHAM gives the worked example of a 10 by 12 foot room, 120 square feet, and says “It would be best to have an air cleaner with a smoke CADR of at least 80.”

EPA’s own chart runs slightly lower, at 65 percent of the area: 100 square feet = 65 cfm, 200 = 130, 300 = 195, 400 = 260, 500 = 325 and 600 = 390. EPA adds: “Note this chart is for estimation purposes.” The gap is small: 9 cfm on a 500-square-foot room.

What the two-thirds rule assumes

The rule has a basis, and knowing it tells you when it stops applying. AHAM says the room size on its label is “calculated based on the removal of at least 80 percent of smoke particles in a steady-state room environment, assuming one air change per hour with complete mixing in the room.” EPA’s Technical Summary likewise sizes for an 80-percent reduction in steady-state particle concentrations.

Those assumptions may not match your room:

• One air change per hour. A room with a window left open may exchange far more air than that, which the math does not allow for.
• Complete mixing. Real rooms have dead corners and furniture.
• An 8-foot ceiling. EPA says “The CADRs are calculated based on an 8-foot ceiling.”
• The top fan speed. The test is performed at the highest setting, and EPA says “An air cleaner will filter less air if it is set at a lower speed.” If you plan to run the unit on low, the label overstates what you will get.

That last point decides many purchases. A unit that exactly meets the minimum only meets it at full volume, so buying above the minimum is what leaves room for a quieter setting.

Air changes per hour, and how to work it out

Air changes per hour (ACH) tells you how many times each hour the room’s whole volume passes through as clean air. CDC/NIOSH gives the formula, which in words is: ACH = (cfm x 60) / (floor area in square feet x ceiling height in feet). For an air cleaner, use the CADR for cfm. CDC calls the result equivalent air changes per hour (eACH) and notes that it “depends on the contaminant.”

For the 180-square-foot room with a smoke CADR of 120 and an 8-foot ceiling:

• 120 x 60 = 7,200 cubic feet per hour
• 180 x 8 = 1,440 cubic feet of room
• 7,200 / 1,440 = 5

So the two-thirds rule and the air-change target are the same thing seen from two sides. On an 8-foot ceiling, two-thirds of the area always works out to 5 ACH, and CDC says that “5 ACH is what portable air cleaners provide (as eACH) when properly sized.” CDC is careful about what that means: it calls 5 ACH “a rough guide to air change levels” and warns that “Five ACH will not guarantee totally safe air in any space.” It is an airflow target and not a health threshold.

To go the other way, from a target ACH to a CADR, multiply the room’s volume by the target and divide by 60. A 1,440-cubic-foot room at 5 ACH needs 1,440 x 5 / 60 = 120 cfm of smoke CADR.

Sizing for wildfire smoke: AHAM and EPA differ

Here the two sources give different answers, and you should know both.

• AHAM says: “For wild fire smoke, AHAM recommends a Smoke CADR equal to the size of the room in square feet.” For the 180-square-foot room, that is a smoke CADR of 180.
• EPA, in its current wildfire material, keeps the same rule as everyday sizing: “Choose a PAC with a clean air delivery rate (CADR) for smoke that is at least two-thirds of the room area in square feet.” For that room, 120. EPA’s June 2026 guide for public health officials adds that portable units should ideally provide “a filtered airflow at least four to five times the room volume per hour.”

If you can reach AHAM’s figure, it gives the room more clean air than the two-thirds rule does. If the room is large or the choice is limited, the two-thirds rule is the one EPA publishes. EPA also says to run the unit continuously on the highest fan setting during smoke events, which is a time when the top-speed CADR actually describes how the unit is being used. The guide to air purifiers and wildfire smoke covers the rest of the preparation, including a clean room. If smoke raises a health concern for anyone in the home, ask a clinician.

Rooms with high ceilings

EPA and AHAM say the same thing in general terms. EPA: “If you have high ceilings (above 8 feet), you should consider using an air cleaner that is sized for a larger area.” AHAM: “If your ceilings are higher than 8 feet, an air cleaner rated for a larger room will be necessary.”

Neither, in the pages used here, prints a correction factor. A simple way to apply the idea is to keep the air changes constant by scaling the area by the ceiling height divided by 8. This is arithmetic and not an official rule:

• 180-square-foot room, 10-foot ceiling: 180 x 10 / 8 = 225 square feet of “equivalent” area, and two-thirds of that is 150.
• Check: 150 x 60 / (180 x 10) = 5 ACH.

Open-plan rooms and connected spaces

The rule treats the room as a closed box. EPA’s Technical Summary says a unit “will be much more effective for a specific room when any exterior doors and windows in a room are closed,” and EPA says portable air cleaners are “designed to filter the air in a single room or area.” Air moves through an open doorway, but not reliably.

How far that reaches is uncertain. In the trial by Lu and colleagues, published in 2024, a HEPA unit running in one room of a home reduced mean PM2.5 by 78.8% in that room and by 57.9% in a second room. Those are air concentrations in 29 homes over two consecutive 24-hour periods, and they say nothing about health. A review by Sublett reports the finding of Macintosh and colleagues from 2008 that “running a single HEPA PRAC in one room did not provide air cleaning in other rooms.” PRAC there means a portable room air cleaner. The two results disagree on how far the effect reaches, but they do not conflict on the main point: expect the largest reduction in the room with the unit.

For an open kitchen and living area, measure the whole connected floor. If the math comes out high, consider the next option.

Why two smaller units can beat one large one

As plain arithmetic, and assuming the air mixes, the CADRs of units in the same space can be added; none of the sources used here prints this as a rule. Two units with a smoke CADR of 130 each give 260, which EPA’s chart matches to a 400-square-foot room. Splitting the job that way has real advantages:

• Distribution. Cleaned air starts from two places, in a space that does not mix well on its own.
• Noise. Two units at a middle setting may be easier to live with than one at top speed, though at a middle setting each delivers less than its rated CADR.
• Large spaces. AHAM says the largest CADR its test can measure is 450, which corresponds to 698 square feet. Past that point, a single rated unit cannot cover the space on paper.

EPA’s 2026 fact sheet on filtration makes the same point for whole homes: “two or more well-placed PACs and/or DIY air cleaners can be just as effective and may cost less than operating a central air system continuously with a high-efficiency filter.”

Placement matters as much as the number. EPA says to keep units away from curtains and anything that might block airflow, and to use them in the rooms where you spend the most time; the guide on where to put an air purifier goes through it room by room.