What is CADR (clean-air delivery rate)?
In one line: CADR is the volume of genuinely clean air a purifier delivers per hour, usually shown in m³/h (or CFM in the US). It's the single most useful spec because it combines how good the filter is and how much air the fan moves — the two things that actually clean a room.
What the number means
A purifier's filter only cleans the air that passes through it, so two figures matter: how much the filter captures, and how much air the fan pushes through it. CADR rolls both into one number — the amount of particle-free air delivered each hour. A unit with a superb filter but a weak fan has a low CADR and cleans a room slowly; a unit with a strong fan and a good filter has a high CADR and turns the room's air over quickly.
You'll often see separate CADR figures for smoke, dust and pollen, because particle sizes differ. Smoke CADR (the finest particles) is the toughest test and the most useful single figure to compare.
How CADR is measured
CADR is measured in a sealed test chamber: particles are introduced, the purifier runs, and the rate at which particle levels fall (beyond natural settling) gives the clean-air delivery rate. Independent testing (for example AHAM-style or Intertek testing) is more trustworthy than a manufacturer's unverified "coverage" claim, because coverage numbers are often marketing estimates rather than measured performance.
How to match CADR to your room
The goal is enough clean air to turn your room over several times an hour (see air changes per hour). A rough rule for a standard 2.4 m ceiling, aiming for ~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 |
Two practical notes: closed rooms need their own unit (air won't clean through walls), and it's better to buy more CADR than you need and run it gently — that's quieter and cleans faster than a small unit at full speed.
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.
FAQ
Is a higher CADR always better? Higher CADR cleans faster and lets you run the unit quietly, but there's 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 and can be optimistic. Compare measured CADR instead, and check the air-change rate it implies for your room.