What is CADR (clean-air delivery rate)?
In one line: CADR is the volume of genuinely clean air a purifier delivers per hour, usually in m³/h (CFM in the US). It is the single most useful spec because it combines how good the filter is and how much air the fan moves — and because, unlike "covers 60 m²", it is a measurement rather than a claim.
What the number means
A filter only cleans the air that actually passes through it. So two things matter: what proportion of particles the filter captures, and how much air the fan pushes through it. CADR combines them:
CADR = airflow × single-pass efficiency
That multiplication is why the spec is so useful, and why filter grade alone tells you very little. A unit with a superb filter and a weak fan delivers a low CADR and cleans a room slowly. A unit with a strong fan and a merely good filter can deliver far more clean air per hour. Both facts are invisible if you only read the filter grade on the box — which is exactly what most product pages invite you to do.
You will often see three CADR figures: smoke, dust and pollen. They differ because particle sizes differ. Smoke is the finest of the three, so smoke CADR is the hardest test and the most useful single number for comparison. A unit that handles smoke well handles dust and pollen comfortably; the reverse is not true.
How CADR is measured
Testing happens in a sealed chamber of known volume. Particles are introduced, the purifier runs, and the rate at which particle concentration falls is recorded. Crucially, the natural decay rate — particles settling out on their own, with the machine switched off — is measured separately and subtracted. What remains is the work the machine actually did, expressed as the equivalent volume of perfectly clean air delivered per hour.
That subtraction is the reason CADR is trustworthy where coverage claims are not. It is also why independent testing matters: an AHAM-style or Intertek-style measurement is a controlled experiment, while a "coverage area" printed on a box is frequently a marketing estimate derived backwards from an assumption nobody states.
The arithmetic that exposes a bad claim
Here is the calculation worth memorising. Clean-air delivery becomes meaningful only when divided by the size of the room it has to clean:
Air changes per hour = CADR (m³/h) ÷ room volume (m³)
Five air changes per hour is the usual domestic target. Now apply it to a typical claim. Suppose a unit is advertised as covering 60 m² and its stated CADR is 300 m³/h. At a 2.4 m ceiling that room holds 144 m³, so:
300 ÷ 144 = 2.1 air changes per hour
Less than half the normal target. The claim is not a lie — the machine will indeed move some air around a 60 m² room — but it quietly assumes a standard of cleaning that few people would accept if it were stated out loud. Run the same division on any coverage claim you are shown and the assumption becomes visible immediately.
This is also the honest explanation for the most common complaint about air purifiers: that they "don't seem to do anything". Very often the machine is working exactly as specified, and the specification was never enough for the room.
How to match CADR to your room
Working forwards instead, for a standard 2.4 m ceiling at 4–5 air changes:
| Room | Floor area | CADR to aim for |
|---|---|---|
| Small bedroom / office | up to 15 m² | 150+ m³/h |
| Double bedroom / lounge | 15–25 m² | 250–350 m³/h |
| Large living room | 25–40 m² | 400–550 m³/h |
| Open-plan / kitchen-diner | 40–60 m² | 550–700 m³/h |
Three practical notes. Closed rooms need their own unit, because air does not clean through walls. In a long or L-shaped space two smaller units usually beat one large one, since clean air has to reach the far end rather than simply existing somewhere in the room. And it is almost always better to buy more CADR than you need and run it gently — which is quieter, and still cleans faster than a small unit working flat out.
Free calculator: CADR for your room
Enter your room and target air changes to get the CADR to aim for — or check whether a specific purifier is powerful enough:
Four things rated CADR does not tell you
It describes a clean filter. As a filter loads, its efficiency creeps up slightly but its airflow falls, and the airflow loss dominates. Real delivery drifts below the rated figure across the filter's life, which is why falling airflow is a better replacement signal than the calendar.
It assumes the unit can breathe. A purifier pushed into a corner, under a shelf or behind a sofa is recirculating its own output and starving its intake. Rated CADR is measured in open air; placement can take a meaningful bite out of it.
It is usually quoted at maximum speed. The figure on the box is typically the noisiest setting. If you intend to run the machine overnight, the number that matters is the CADR at the speed you will actually use — which is why a unit with plenty of headroom beats one that must run flat out.
It says nothing about gases. CADR is a particle measurement. Odours, cooking smells and VOCs are molecules, not particles, and need activated carbon. A high-CADR particle filter will do nothing measurable to a smell.
Real UK figures: measured CADR, noise & running cost
Reference figures for popular portable units, with running cost estimated at a UK rate of ~£0.25/kWh running 12 hours a day:
| Model | CADR | Noise (quiet setting) | Power | Est. running cost/yr* |
|---|---|---|---|---|
| Luggable 5-Fan | 369 m³/h | under 35 dB | 6–16 W | ~£7–18 |
| Luggable XL 7-Fan | 622 m³/h | under 35 dB | 8–20 W | ~£9–22 |
| Typical HEPA tower | 300–400 m³/h | 50–60 dB on high | 40–70 W | ~£44–77 |
*Our estimate from published wattage and current UK electricity pricing — a low-wattage MERV-13 unit costs a fraction of a mains-hungry HEPA tower to run. A DIY Corsi-Rosenthal box reaches similar CADR for well under £100 to build.
Note what the wattage column does to the comparison. The HEPA tower delivers less clean air while drawing three to four times the power, because forcing air through dense HEPA media costs energy. Clean-air delivery per watt is the figure that decides what a machine costs you over a decade, and it rarely appears on any box.
Why CADR matters more on high-pollution days
When outdoor air is poor, indoor levels follow it upward, because homes leak. The purifier is then competing against a continuous inward flow rather than cleaning a fixed quantity of air once — and the higher the CADR, the higher the pollution level at which the machine can still hold the room down. Here is the live UK reading:
Next step
Want units compared on measured CADR (plus noise and running cost)? See Best Air Purifier UK, or read how the high-airflow Corsi-Rosenthal design achieves a high CADR cheaply.
Looking for a unit that does this well? The Luggable XL is #1 of 247 on the US ENERGY STAR Most Efficient list, lab-tested by Intertek, and uses standard 3M MERV-13 filters.
FAQ
Is a higher CADR always better? Higher CADR cleans faster and lets you run the unit quietly, but there is no point paying for far more than your room needs. Match it to your room, then value low noise and running cost.
CADR vs "coverage area"? Coverage claims are often based on a low air-change rate. Divide the stated CADR by the claimed room's volume and check the implied air changes per hour before believing it — the arithmetic above shows how far apart the two can be.
Which figure should I compare if only one is given? Smoke CADR. It is the hardest of the three tests, so it is the one a weak machine cannot hide behind.
Does a bigger room just need proportionally longer? Not quite — a purifier fights a continuous inward leak of outdoor air as well as cleaning what is already inside, so below a certain CADR a large room never really comes down, however long you run it.
How does CADR relate to HEPA vs MERV-13? Filter grade sets the efficiency term in the equation; the fan and the filter area set the airflow term. A lower-grade filter with far more airflow can and often does win on delivered clean air.