CADR (clean air delivery rate) is the volume of particle-free air a purifier effectively delivers per unit of time. It combines airflow and filter efficiency into one number and is measured in a test chamber by comparing how fast particles disappear with the purifier on versus off. Divide CADR (m³/h) by your room volume (m³) to get clean air changes per hour. CADR says nothing about noise, gases (unless a separate gas CADR is given) or performance with a loaded filter.
Why CADR exists
Air purifier specifications used to list airflow and filter efficiency separately, which made comparisons almost impossible. A unit moving 500 m³/h through a mediocre filter might remove fewer particles than one moving 300 m³/h through an excellent filter. CADR solves this by measuring the result — how quickly the device actually removes particles from a room-sized volume of air.
How CADR is measured
The most widely used method is ANSI/AHAM AC-1, the basis for CADR labels in North America. In outline:
- The purifier is placed in a sealed test chamber of about 28.5 m³ (1,008 ft³), roughly the size of a small bedroom, with a mixing fan to keep the air uniform.
- A test aerosol is released — tobacco smoke, fine test dust or pollen — and its concentration is measured over time.
- The natural decay rate is measured with the purifier off: particles settle on surfaces and leak out slowly.
- The test is repeated with the purifier running at the tested setting.
- CADR is calculated from the difference between the two decay rates, multiplied by the chamber volume.
Smoke, dust and pollen: three CADR values
Filters and airflow paths behave differently across particle sizes, so AHAM reports three values:
| CADR type | Approximate particle size | Represents | Use it for |
|---|---|---|---|
| Smoke | ≈ 0.09–1.0 µm | Fine particles similar to combustion aerosol and much of PM2.5 | Default sizing figure; usually the most conservative |
| Dust | ≈ 0.5–3 µm | Household dust fractions, some mould spores | General household particles |
| Pollen | ≈ 5–11 µm | Larger particles that also settle quickly on their own | Seasonal allergy use; less demanding for filters |
If only one number is shown without saying which particle it refers to, ask. A pollen CADR can look impressive because large particles are easy to capture and also settle naturally.
Other CADR systems
China's GB/T 18801 standard also uses CADR, reported directly in m³/h, and — importantly — includes a separate CADR for gaseous pollutants such as formaldehyde. European and other manufacturers often quote CADR measured to AHAM or GB/T methods, or their own internal tests. When comparing two products, check that both values come from the same method.
Converting units
| From | To | Multiply by |
|---|---|---|
| CFM (ft³/min) | m³/h | 1.699 |
| m³/h | CFM | 0.589 |
| L/s | m³/h | 3.6 |
| m³/h | L/s | 0.278 |
Example: a smoke CADR of 250 CFM equals about 425 m³/h, or about 118 L/s.
From CADR to your room
For a full sizing method and a quick-reference table, see how to size an air purifier for a room, or try the calculator.
What CADR does not tell you
- Noise at that setting. CADR is usually measured at maximum speed. The unit may be too loud to run there. Ask for CADR and sound level at each speed — see air purifier noise explained.
- Performance with a loaded filter. Tests use new filters. As filters load, many fans deliver less air.
- Gas and odor removal. Particle CADR does not measure carbon performance. Look for a separate gaseous CADR if odors or VOCs matter.
- Your room's mixing. The test chamber is actively mixed. Obstructed or oddly shaped rooms may see lower effective performance — see how airflow affects indoor air quality.
- Energy efficiency. Two units with the same CADR can differ considerably in power draw. CADR per watt is a useful secondary metric.
Where to find trustworthy CADR values
Third-party certification programmes, such as AHAM Verifide in North America, publish CADR values that have been independently tested. For products outside such programmes, ask the manufacturer for the test report: the standard used, the laboratory, the tested fan setting and whether it was the complete device. A CADR claim with no method is not comparable with anything.
Key takeaways
- CADR is effective clean air delivered — airflow multiplied by filter efficiency — measured in a test chamber.
- Smoke CADR is the best default for sizing; pollen CADR is the least demanding.
- Divide CADR (m³/h) by room volume (m³) to get clean air changes per hour.
- Ask for CADR and noise at the speed you will actually use, and the test method behind the number.
Frequently asked questions
What is a good CADR?
A good CADR is one that matches your room. Multiply room volume in m³ by your target air changes per hour: a 40 m³ bedroom at 5 ACH needs about 200 m³/h (roughly 120 CFM). A very high CADR in a small room mainly lets you run the unit on a quieter setting.
Why does a purifier list three different CADR values?
The AHAM AC-1 method tests with tobacco smoke, fine dust and pollen because filters and airflow behave differently across particle sizes. Smoke CADR covers the smallest particles and is usually the most conservative figure to use for sizing.
Is CADR measured at the highest fan speed?
Usually, yes. Headline CADR figures are typically measured at maximum speed with a new filter. Ask for the CADR or airflow at the speed you expect to use day to day, together with the noise level at that speed.
Sources and further reading
- ANSI/AHAM AC-1 — Method for measuring performance of portable household electric room air cleaners
- AHAM Verifide — room air cleaner certification programme
- GB/T 18801 — Air cleaner (Chinese national standard, CADR reported in m³/h for particles and formaldehyde)
- U.S. EPA — Guide to Air Cleaners in the Home