CADR stands for Clean Air Delivery Rate — a tested measure of how much filtered air a purifier delivers, in cubic feet per minute. It’s reported separately for smoke, dust, and pollen. A higher CADR means the unit cleans a given room faster, which is why CADR is matched to room size.
What does the CADR number measure on an air purifier?
CADR, or Clean Air Delivery Rate, measures how much clean air a purifier delivers in cubic feet per minute. It combines the fan's airflow with the filter's efficiency into one tested figure, so a higher CADR means faster particle removal in a given space.
Because it reflects real delivered clean air, CADR is the most useful single number for comparing how quickly two purifiers can refresh the indoor air quality of the same room. AHAM, the industry association behind the test, certifies CADR ratings so the numbers on the box are comparable across brands.
Why are there three CADR numbers — smoke, dust, and pollen?
CADR is tested separately for smoke, dust, and pollen because these particles differ in size. Smoke is the smallest and hardest to capture, so the smoke CADR is usually the lowest of the three and the most demanding benchmark to compare units by.
When matching a purifier to a room, the smoke rating is the safest figure to size around, since a unit that handles fine smoke particles will comfortably manage larger dust and pollen too.
How does CADR relate to a room's square footage and air changes per hour?
A common guideline is the two-thirds rule: the smoke CADR should be at least about two-thirds of the room's square footage. This roughly ensures the purifier can deliver enough air changes per hour for a healthy indoor climate.
Higher ceilings, open floor plans, and heavy pollution all argue for extra CADR beyond the minimum, because the rule assumes a standard ceiling height and a reasonably enclosed room. Aiming for four to five air changes per hour is a common target for allergy or smoke-sensitive households.
What specs besides CADR should you check?
CADR describes speed, but the filter stack determines what actually gets captured and how often you'll pay to maintain it.
True HEPA and activated carbon filters
A true HEPA filter captures at least 99.97% of particles down to 0.3 microns, the benchmark the EPA references for HEPA-grade filtration; some budget units use "HEPA-type" media that falls short of this. Activated carbon adds a separate layer that adsorbs gases and odors, which raw particle filtration alone can't touch.
Pre-filter and filter replacement
A washable pre-filter catches larger debris like hair and lint before it reaches the true HEPA layer, extending its life. Check the filter replacement cost and interval before buying, since a cheap purifier with expensive, frequent filter replacement can cost more over a year than a pricier unit with cheaper filters.
What does CADR miss — PM2.5, VOC, and allergens?
CADR is a lab-tested airflow number, not a live pollutant reading, so it says nothing about how well a purifier handles PM2.5, VOC gases, or specific allergens in your actual home. A high CADR with only a particle filter still won't touch VOC odors from cooking or paint, since that needs the activated carbon layer.
For allergens like pet dander and pollen, true HEPA filtration paired with adequate CADR for the room is what matters; a model's real-time PM2.5 sensor, where included, gives a more direct read on current air quality than the static CADR spec alone.
What daily-use features round out a good purifier?
Beyond CADR and filtration, a few features shape how much you actually use the unit.
Quiet operation and auto mode
A purifier that's quiet enough for a bedroom on its low setting gets left running; a loud one gets switched off, defeating the CADR rating entirely. Auto mode, paired with a particle sensor, ramps the fan up during a pollution spike and back down once the air is clean, balancing quiet operation with performance.
Energy efficient models and air quality
An energy efficient purifier costs little to run continuously, which matters since air quality benefits most from near-constant operation rather than occasional bursts. LEVOIT, Coway, Winix, and Blueair all publish CADR, filter life, and wattage specs, making side-by-side comparison straightforward once you know which numbers to check.
How do you calculate the CADR you actually need?
Take the room's square footage, multiply by roughly two-thirds, and look for a smoke CADR at or above that number — a 300 sq ft bedroom points to a smoke CADR near 200. Round up if the ceiling is tall or the room is open to a hallway or kitchen.
Manufacturer "coverage area" claims printed on the box sometimes assume a lower target of one or two air changes per hour, which is thinner than what allergy or smoke-sensitive households actually want. Checking the underlying smoke CADR number yourself, rather than trusting the coverage claim alone, gives a more honest picture of how the purifier will perform against dust, pollen, and fine PM2.5 particles in your specific room.
Match CADR to the room rather than chasing the biggest number. For sizing help, see the air purifiers hub, the large-room purifiers hub, and how we rank.