Air Purifier Sizing Guide: What CADR Do You Actually Need?

Buying an air purifier that is too small for your room is the most common mistake we see. An undersized purifier runs constantly but never cleans the air fast enough — it is like running a garden hose to fill a swimming pool. Here is exactly how to size an air purifier correctly.

The CADR Rule: Your Starting Point

CADR (Clean Air Delivery Rate) measures how much filtered air a purifier delivers per minute, measured in cubic feet per minute (CFM). Every purifier is tested for three particle types: Smoke, Dust, and Pollen.

The 2/3 rule: Your purifier's Smoke CADR should be at least two-thirds of your room's square footage.

Why Air Changes Per Hour Matter

CADR tells you flow rate, but ACH (Air Changes Per Hour) tells you how often your entire room volume passes through the filter. For meaningful air quality improvement, you need:

• 2 ACH — Minimum for noticeable improvement
• 4 ACH — Recommended for allergy relief
• 5+ ACH — Ideal for asthma or severe allergies
• 6+ ACH — Clinical/medical-grade filtration

How to Calculate ACH

ACH = (CADR x 60) / (Room Area x Ceiling Height)

For a typical room with 8-foot ceilings:

The sweet spot is highlighted: cells with 4+ ACH deliver reliable allergy relief in our testing.

Room-by-Room Recommendations

Small Bedroom (under 200 sq. ft.)

Best pick: Levoit Core 300 — $100

At 141 CADR, the Core 300 delivers 5.3 ACH in a 200 sq. ft. bedroom. That is above the 4 ACH threshold for allergy relief. Its 24 dB noise level on low makes it ideal for sleep.

Do not overspend here. The Coway Mighty or Blueair 211+ would be overkill in a small bedroom — they clean the air faster, but you are paying for capacity you do not need.

Medium Bedroom or Office (200-350 sq. ft.)

Best pick: Coway Airmega AP-1512HH — $229

The Coway Mighty is the standard recommendation for medium rooms. Its 233 CADR achieves 5.8 ACH in a 300 sq. ft. room — comfortably above the allergy relief threshold.

Budget alternative: Winix 5500-2 ($160) delivers virtually identical CADR with lower operating costs.

Large Living Room (350-500 sq. ft.)

Best pick: Blueair Blue Pure 211+ — $300

For large rooms, you need raw airflow. The Blueair 211+'s 350 CADR achieves 5.2 ACH in a 500 sq. ft. space. No other purifier under $350 matches this.

Alternative: Honeywell HPA300 ($250) provides 300 CADR at a lower price, but with higher noise and operating costs.

Open Floor Plans (500+ sq. ft.)

For spaces over 500 square feet, a single consumer purifier typically cannot maintain 4+ ACH. Two strategies:

1. Two medium purifiers — Place a Coway Mighty at each end of the space. Two units at 233 CADR each effectively deliver 466 CADR across the space.

2. One large purifier + one small — Blueair 211+ in the main area + Levoit Core 300 in the adjacent space.

Two purifiers generally outperform one large unit because they distribute clean air more evenly across the space.

Common Sizing Mistakes

Mistake 1: Using the manufacturer's coverage rating

Manufacturer coverage ratings often assume 2 ACH — barely enough for noticeable improvement. For allergy relief, you need 4+ ACH, which means the effective room size is roughly half the manufacturer's rated coverage.

Mistake 2: Sizing for the whole house

Air purifiers clean one room effectively. Closing the door dramatically improves performance because particles cannot drift in from adjacent rooms. If you want clean air in three rooms, you need three purifiers.

Mistake 3: Running on low speed to save noise

Running a purifier on low reduces its effective CADR significantly. If noise is a concern, buy a larger purifier and run it on low rather than buying a smaller purifier and running it on high. The larger unit on low will deliver more clean air at less noise.

Quick-Reference Sizing Chart

Our Top Pick for Most Rooms

For the majority of bedrooms and offices (200-350 sq. ft.), the Coway Airmega AP-1512HH provides the best balance of CADR, noise, and value: