Air Purifier CADR Rating Explained: What It Means and How to Use It

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What CADR Actually Measures

CADR stands for Clean Air Delivery Rate. It is published in cubic feet per minute (CFM) and answers one question: how many cubic feet of clean air does this purifier produce per minute?

A unit with a CADR of 200 for dust delivers 200 cubic feet of dust-free air per minute. That is all it means. It is not a coverage claim, not a square footage recommendation, and not an efficiency rating. It is a flow rate.

CADR is tested and published by AHAM (the Association of Home Appliance Manufacturers) using a standardized protocol. The tests run in a controlled 1,008 cubic foot room (roughly 8'x12'x10.5'). The unit runs for 20 minutes and the before/after particle reduction is measured.

Why CADR Has Three Numbers

AHAM tests three particle types separately because they have different sizes and behave differently in air:

**Smoke CADR**: targets particles 0.09-1 micron (fine combustion particles, the hardest to filter)
**Dust CADR**: targets particles 0.5-3 microns (household dust, dust mites, pollen)
**Pollen CADR**: targets particles 5-11 microns (larger airborne particles)

A unit can score differently across all three. Some cheaper purifiers score high on pollen (large, easy to catch) but low on smoke (small, harder). For allergy and asthma sufferers, the dust CADR is the most relevant because the particle sizes that trigger most reactions cluster in the 0.5-3 micron range.

**For most buyers:** look at the dust CADR as your primary number.

How to Calculate Room Coverage

The formula: to find the maximum room size a purifier covers at 2 ACH (air changes per hour), multiply CADR by 1.55.

Example: a purifier with dust CADR of 200 covers 200 × 1.55 = 310 sq ft at 2 ACH (8 ft ceiling assumed).

For 3 ACH (better for allergy/asthma use): divide that number by 1.5. So 310 / 1.5 = 207 sq ft.

For 5 ACH (recommended for COVID risk reduction): divide by 2.5. So 310 / 2.5 = 124 sq ft.

Most manufacturer "coverage" claims on boxes use the 2 ACH standard with 8 ft ceilings. That is the best-case number. For real-world allergy or health use, assume the effective coverage is 50-70% of what the box says.

Quick reference table:

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What CADR Does Not Tell You

CADR measures how fast the unit produces clean air. It does not measure:

**Filter grade.** A CADR of 200 from a True HEPA filter and a CADR of 200 from a HEPA-style filter are not equivalent. The filter grade determines what particle sizes are captured. CADR tells you the flow rate, not the filtration quality.

**Noise level.** High CADR can be achieved by running the fan fast and loud. A unit with 300 CADR at 65 dB is not better than a unit with 280 CADR at 40 dB for most home use — you will run the loud one on a lower setting anyway, reducing effective CADR.

**Carbon filtration.** CADR measures particle removal. Gas molecules (VOCs, odors) are not particles and are not measured by CADR. A high CADR unit with a thin carbon layer will not perform better than a low CADR unit with a thick carbon bed for cooking odors or chemical off-gassing.

**Long-term performance.** CADR is tested on a new filter. A 6-month-old HEPA filter with high particle loading has lower effective CADR because airflow is reduced. The rating reflects new-filter performance only.

The Manufacturer "Coverage" Claim Problem

If a box says "covers up to 1,000 sq ft" and the dust CADR is 300, the math does not add up at any useful ACH. 300 CADR × 1.55 = 465 sq ft at 2 ACH. 1,000 sq ft would require a CADR of around 645.

How do manufacturers justify these numbers? Some claim coverage at 1 ACH (not useful). Some use room volumes smaller than 8 ft ceilings. Some simply inflate the number. Always calculate from CADR directly rather than trusting the box coverage claim.

Bottom Line

CADR is the most useful single number for comparing air purifiers. Use the dust CADR as your primary metric. Calculate effective coverage by multiplying CADR by 1.55 for 2 ACH or dividing that by 1.5 for 3 ACH. For allergy/asthma use, target 3 ACH. For COVID risk reduction, target 5 ACH. Ignore box coverage claims and do the math from the published CADR.

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