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What CADR actually measures
CADR stands for Clean Air Delivery Rate. It is measured in cubic feet per minute (CFM) and represents the volume of air the purifier cleans of a specific pollutant each minute.
It is not the same as airflow. A purifier might move 300 CFM of air but only deliver a CADR of 200, because some air passes through without being fully filtered. CADR is the effective number — the air that actually comes out clean.
CADR is set by AHAM (the Association of Home Appliance Manufacturers) and independently tested, which is why it is the one spec you can trust. Room-size claims are not independently verified.
The three CADR numbers
Every certified purifier publishes three CADR figures, one for each pollutant type:
| CADR type | Pollutant | Typical use |
|---|---|---|
| Smoke CADR | Smoke, PM2.5 | The headline number — use this one |
| Dust CADR | Dust, pollen | Usually similar to smoke |
| Pollen CADR | Pollen, allergens | Usually the highest of the three |
When comparing purifiers, use the smoke CADR. It is the lowest of the three and the best indicator of real-world performance, because smoke particles are the hardest to capture.
Sizing a purifier with CADR
The AHAM rule is that a purifier should clean the room's air 4.8 times per hour. That gives a simple formula:
Required CADR = room area (sq ft) × 0.8 (for a standard 8-foot ceiling).
| Room size | Minimum smoke CADR |
|---|---|
| 150 sq ft | 120 |
| 250 sq ft | 200 |
| 400 sq ft | 320 |
| 600 sq ft | 480 |
| 900 sq ft | 720 |
In practice, buy a unit rated for 1.5 times your room size. That way it runs on a lower, quieter speed and still keeps up.
Why the room-size number on the box is not the spec
Manufacturers print a room size on the box, but it is derived from CADR using assumptions you cannot see — often a lower air-change rate, or a ceiling height that is not yours. Two purifiers with the same claimed room size can have very different CADRs.
- Always compare CADR, not room size. The CADR is measured; the room size is marketing.
- Check the ceiling height. A room-size claim assumes 8-foot ceilings. A lofted or high-ceiling apartment needs more CADR.
- Check the noise at that CADR. A unit may only reach its rated CADR on high, which may be too loud to use.
Four CADR mistakes
- Comparing room size instead of CADR. Room size is derived from CADR using assumptions you cannot see. Compare the CADR.
- Using the pollen CADR. Pollen CADR is the highest of the three and flatters the unit. Use the smoke CADR, which is the lowest and most realistic.
- Ignoring noise at that CADR. A unit may only reach its rated CADR on high, which may be too loud to use. Check the noise at the speed you will actually run.
- Buying far larger than needed. An oversized unit wastes money on filters. Aim for 1.5 times your room size, not five times.
CADR vs other specs
CADR is not the only number on the box, but it is the only one that is independently measured. Here is how the others compare:
| Spec | Measured? | Useful? |
|---|---|---|
| CADR | Yes, by AHAM | Yes — the number to compare |
| Room size | No — derived | Only as a rough guide |
| Airflow (CFM) | Sometimes | Less useful than CADR |
| Filter type | No | Yes — check for true HEPA |
| Noise (dB) | Sometimes | Yes — check at the speed you use |
A purifier can have a high airflow and a low CADR if air leaks around the filter instead of passing through it. That is why CADR, not airflow, is the number that tells you how well the unit actually cleans.
The room-size math
CADR converts to room size with a simple rule: divide the CADR by 1.55 to get the square footage for four air changes per hour, or by 0.8 for the CADR you need for a given room.
| Room area | CADR for 4 ACH | CADR for 5 ACH |
|---|---|---|
| 150 sq ft | 120 | 150 |
| 250 sq ft | 200 | 250 |
| 350 sq ft | 280 | 350 |
| 500 sq ft | 400 | 500 |
| 700 sq ft | 560 | 700 |
The 4-ACH figure is the standard used by most manufacturers. For allergy sufferers or a room with a smoker, aim for 5 ACH. And buy 1.5 times the minimum so the unit runs on medium rather than high, which is quieter and cheaper to run.
The three CADR numbers
Every AHAM-certified purifier publishes three CADR figures, and they are not interchangeable.
| CADR type | Typical value | Use it? |
|---|---|---|
| Smoke | Lowest | Yes — the realistic number |
| Dust | Middle | Sometimes |
| Pollen | Highest | No — flatters the unit |
Pollen particles are large and easy to catch, so the pollen CADR is always the highest. Smoke particles are the smallest and hardest, so the smoke CADR is the lowest and the most honest. When you compare two purifiers, compare the smoke CADR.
What CADR does not tell you
CADR measures particle removal only. It says nothing about odour, VOCs or CO2, and it says nothing about noise. A purifier can have a high CADR and be too loud to use, or have a high CADR and no carbon at all. Use CADR to compare particle performance, then check the carbon weight and the noise spec separately.
Choosing by CADR
Once you know the CADR you need, choosing is simple. Compare the smoke CADR, then check the carbon and the noise.
- Measure the room and multiply the area by 0.8 for the minimum CADR.
- Multiply by 1.5 so the unit runs on medium, not high.
- Compare the smoke CADR of the units in your budget.
- Check the carbon weight if you need odour control.
- Check the noise at the speed you will actually use.
A unit that meets your CADR target on medium is the right buy. A unit that only reaches it on high will be too loud to use, and you will end up running it lower than you need.
The mistake to avoid
The most common mistake is buying by room size instead of CADR. Room size is derived from CADR using assumptions you cannot see, and manufacturers are generous with it. Compare the CADR, and use the smoke figure — it is the lowest and the most honest of the three.
Frequently asked questions
What does CADR mean on an air purifier?
What is a good CADR for an air purifier?
Is higher CADR always better?
What is the difference between CADR and airflow?
Which CADR number should I use to compare purifiers?
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