Match CADR to room size: coverage at 2, 4.5, and 6 air changes per hour

Understanding the three air change rates

CADR is the Clean Air Delivery Rate in cubic feet per minute, measured under the AHAM AC-1 standard in a sealed test chamber. To translate CADR to a room size, you need to know how many times per hour the air should be completely replaced.

Air change rate, abbreviated ACH, is the number of times per hour that all the air in a room is exchanged. At 2 ACH, the air is completely replaced twice per hour, or every 30 minutes. At 6 ACH, the air is replaced six times per hour, or every 10 minutes.

The difference between these rates matters: two air changes per hour is the minimum to reduce allergens, pollen, and dust mites over a day. Four and a half to five air changes per hour is the standard for allergy households, recommended by the American College of Allergy, Asthma and Immunology. Six air changes per hour or higher is used for smoke or wildfire events.

Which air change rate is right for you

• 2 ACH: General air quality maintenance in a bedroom or office with no specific pollution source.
• 4.5 ACH: Allergy, asthma, or dust mite households, or regular indoor cooking smoke.
• 6 ACH or higher: Wildfire smoke, active cooking smoke, or severe indoor pollution.

How to use this chart to size a purifier

Step one: measure your room floor area in square feet. Multiply length times width. If the room is not rectangular, divide it into rectangles.

Step two: decide which air change rate you need based on pollution sources. If you have pets and allergies, use 4.5 ACH. If you cook regularly or live near wildfires, use 4.5 to 6 ACH.

Step three: find the row in the chart where the ACH column matches your target, and read across to find the closest room size. The CADR column on the left tells you the minimum CADR you need.

For example, a 300 square foot bedroom with allergy concerns (4.5 ACH) needs a purifier with at least 200 CADR. A 500 square foot living room with regular cooking needs 300 CADR at 4.5 ACH.

The math behind CADR to room size

The formula is straightforward: Room size in square feet = (CADR in CFM × 7.5) divided by ACH.

This comes from the definition of air change rate. If a room has one air change per hour (1 ACH), the purifier delivers a volume of air equal to the room size, every hour. For an 8-foot ceiling, that volume is floor area (sq ft) times 8 (ft). At 60 minutes per hour, CFM equals floor area times 8 divided by 60. Rearranging: floor area = CFM times 60 divided by 8, which simplifies to CFM times 7.5.

Divide by the target ACH, and you get the room size the CADR can support at that rate. This arithmetic matches the calculators on the site.

What happens if you underscore CADR

Underscoring CADR means the purifier delivers fewer air changes per hour than intended. At 3 ACH instead of 4.5, the effective room coverage shrinks by a third. The purifier still removes particulate, but it takes longer to reach steady state.

In a real room with continuous pollution (cooking, smoke), undersized purifiers fall behind. The indoor PM2.5 does not drop as fast, and on heavy pollution days, it climbs faster than the purifier can clear it.

Oversizing CADR is cheap insurance. A 200 CADR unit in a 200 square foot room at 4.5 ACH delivers 6 ACH instead, clearing the air twice as fast. The cost is higher energy use, which is why running on low speed saves money if AQI is moderate.

Multiple units in larger spaces

A 1,200 square foot open living area cannot be served by a single 200 CADR purifier at 4.5 ACH (which covers 333 sq ft). You need either a single 900+ CADR unit, or two to three medium units placed apart.

Two smaller units placed on opposite ends of the room will maintain lower concentrations than one large unit in a corner, because they distribute purified air more evenly. The standard practice in large spaces is to run multiple units rather than one oversized unit.