CADR stands for clean air delivery rate, and it is a volume rather than a percentage.
A filter grade tells you how much of the air passing through gets cleaned. CADR tells you how much air passes through.
The two numbers together decide the outcome. Either one on its own tells you almost nothing.
CADR is measured in cubic metres per hour, and the size of it is the whole point
On a British box the figure is usually m³/h. American boxes quote CFM, cubic feet per minute.
To convert, multiply CFM by 1.7. A 250 CFM machine is roughly 425 m³/h.
The test behind the number is AHAM AC-1, run in a sealed chamber of about 28 m³ with the fan at maximum.
Particles are injected, the rate at which they disappear is measured, and the rate at which they would have settled on their own is subtracted.
What survives that subtraction is the machine's own contribution. That is what gets printed.
Three figures, because three particle sizes behave differently
A properly tested purifier carries three CADR numbers: smoke, dust and pollen.
Smoke covers roughly 0.09 to 1 micron. Dust covers 0.5 to 3. Pollen covers 5 to 11.
Pollen is always the highest of the three, because large particles are easy to catch and fall out of the air unaided.
Smoke is always the lowest, and it is the one worth reading.
If a manufacturer quotes a single unlabelled CADR, assume it is the pollen figure until proved otherwise.
A machine advertising 400 m³/h on pollen and 220 on smoke is a 220 machine for everything that actually bothers you.
Pollen CADR sells the machine. Smoke CADR is the one that has to live in your sitting room.
Turn CADR into a room size with two multiplications
Take the floor area, multiply by the ceiling height, and you have the room volume in cubic metres.
A typical British sitting room of 20 m² with a 2.4 m ceiling holds 48 m³.
Then decide how many times an hour you want that volume replaced. Five changes is the usual target for allergy and asthma households.
Five changes of 48 m³ is 240 m³/h, and that is the CADR you are shopping for.
Two changes an hour is enough for general dust and cooking smells, which drops the requirement to 96 m³/h.
Smoke CADR needed at five air changes an hour, assuming a 2.4 m ceiling.
Manufacturers quote a room size themselves, and it is usually generous.
Check which air-change rate they used to arrive at it. Many use two an hour and do not say so anywhere on the box.
The printed figure is measured at full fan speed, which is not how anyone runs one
CADR is a maximum-speed number. At maximum speed most purifiers sit somewhere around 50 to 60 dB.
That is conversation volume. Nobody lives with it in the evening, and most owners discover this in week one.
On the quietest setting the same machine may deliver a third of its rated CADR.
So the sensible approach is to buy more CADR than the room strictly needs and then run it slowly.
An oversized purifier on speed one is quiet, effective and cheap to run. A precisely sized one on speed four is none of those things.
Filter resistance also drops airflow as the cartridge loads, so real delivery falls steadily between filter changes.
We work through what those figures look like on one specific machine in our Blueair Blue Pure 311i Max review.
One last limit worth knowing: CADR is not a measure of gases.
Cooking smells, solvent fumes and VOCs are the carbon layer's job, and AHAM does not test for them at all.
A purifier with a superb CADR and a token carbon filter will clear the dust and leave the smell exactly where it was.
Look up a purifier's certified CADRAHAM Verifide, the body that runs the test →
