How to Choose the Right Air Purifier Size

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The Core Formula

CADR (Clean Air Delivery Rate) is the sizing metric for air purifiers. It tells you how many cubic feet of clean air the unit delivers per minute.

The basic coverage formula: **CADR × 1.55 = square feet at 2 air changes per hour (ACH)**

2 ACH means the unit cleans the room's entire air volume twice per hour — the standard used in most manufacturer coverage ratings.

For allergy sufferers, people with asthma, or for COVID risk reduction, you want more air changes: 4-5 ACH. At 4 ACH, your coverage formula becomes: **CADR ÷ 1.3 = square feet at 4 ACH**. At 5 ACH (COVID-level mitigation): **CADR ÷ 1.6 = square feet**.

The most common sizing mistake: buying a unit based on the 2 ACH coverage number when your situation actually calls for 4-5 ACH, resulting in under-filtered air.

Step 1: Measure the Room

Multiply length × width of the space. For irregularly shaped rooms, estimate by area. For open-plan spaces (kitchen open to living room), use the combined square footage — the air purifier has to handle the whole connected volume.

Standard room sizes for reference:

Small bedroom: 100-150 sq ft
Standard bedroom: 150-220 sq ft
Primary bedroom: 220-350 sq ft
Living room: 250-500 sq ft
Open-plan kitchen/dining/living: 400-700 sq ft

Step 2: Determine Your Target ACH

**2 ACH**: General air quality improvement, light dust and odor control. Adequate for otherwise healthy adults with no specific respiratory concerns.

**4 ACH**: Recommended for allergy sufferers, people with asthma, homes with multiple pets, or anyone who wants meaningful particulate reduction. The AAFA recommendation for allergy management.

**5 ACH**: COVID-level risk mitigation per CDC guidance. Also appropriate for wildfire smoke management.

If you have pets, allergies, or are immunocompromised, target 4 ACH as your baseline. Do not size for 2 ACH and hope it is enough.

Step 3: Calculate Required CADR

Once you have your room size and target ACH:

For 2 ACH: required CADR = sq ft ÷ 1.55
For 4 ACH: required CADR = sq ft × 1.3
For 5 ACH: required CADR = sq ft × 1.6

**Examples:**

200 sq ft bedroom, 2 ACH: need CADR 130 (any budget unit covers this)
200 sq ft bedroom, 5 ACH (COVID mitigation): need CADR 320 (need a real purifier, not the cheapest option)
400 sq ft living room, 4 ACH: need CADR 520 (need a premium unit; Coway Airmega 400 at 400 CADR is still short)
500 sq ft open plan, 2 ACH: need CADR 323 (Winix 5500-2 at 360 CADR covers this)

Step 4: Account for Room Conditions

Several factors push the required CADR higher than the formula suggests:

**Ceiling height**: The formula assumes standard 8-foot ceilings. Vaulted or 9-10 foot ceilings increase air volume by 12-25%. Add that percentage to your required CADR.

**Multiple pets**: Each dog or cat in a household adds meaningful dander load. If you have two or more pets, add 20-30% to your CADR requirement over what the formula gives.

**Smokers in the home**: Tobacco smoke is among the hardest pollutant loads for a purifier to handle. In smoking households, size for 5 ACH minimum.

**Proximity to highways or industrial areas**: Elevated outdoor PM2.5 infiltrates through the building envelope. If your outdoor air quality index is regularly over 100, size for 4-5 ACH even without indoor sources.

Step 5: Check the CADR Spec Correctly

Air purifier CADR ratings have three values: smoke, dust, and pollen. These are different because particles of different sizes have different filtration efficiency. Smoke CADR is the most useful for general sizing — it tests the smallest particles (where HEPA filters are least efficient).

Some manufacturers advertise coverage numbers ("covers 400 sq ft!") based on smoke CADR at 2 ACH. Others use the highest of the three CADR numbers. Some use a non-AHAM certified self-reported number. Check whether the CADR is AHAM (Association of Home Appliance Manufacturers) certified — AHAM-certified CADR numbers use a standardized test protocol and are reliable for comparison. Non-certified CADR numbers may be inflated.

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