What Is CADR, and How Much Do You Need?

CADR is measured by running a purifier in a sealed test chamber seeded with a pollutant, then tracking how fast concentration falls relative to natural settling. The result is expressed as the volume of perfectly clean air the machine effectively delivers per hour.

That framing is what makes it useful. A purifier with a superb filter and a weak fan and a purifier with a mediocre filter and a powerful fan can produce the same CADR, and for your room, they perform the same. It collapses two specifications into the one that matters.

Air changes per hour is the figure you actually want. Calculate your room volume — floor area in m² times ceiling height, usually 2.7 m in Indian flats — then divide CADR by that volume.

A 150 sq ft bedroom is about 14 m², so roughly 38 m³ of volume. A purifier rated 200 m³/h gives you 200 ÷ 38 ≈ 5.3 air changes per hour, which is a good target for a polluted city.

• It is measured at maximum fan speed. The CADR you get at the setting you actually run at night is a fraction of the rated figure — sometimes a third of it.
• It is measured on a new filter. A HEPA at nine months in an Indian city delivers materially less, because airflow falls as the filter loads.
• It is measured in a sealed chamber with no continuing pollution source. Your flat has doors, windows, a kitchen and a street, all adding load continuously.

Formal CADR testing measures three separately: smoke (the smallest particles, roughly 0.09-1 micron), dust (0.5-3 microns) and pollen (5-11 microns). Where all three are published, the smoke figure is the most demanding and the most relevant to PM2.5.

Most Indian listings publish a single number without saying which. Where a brand publishes all three, that is itself a small signal about how seriously it takes specification. Where a brand publishes none, be sceptical — a purifier unwilling to state its CADR is asking to be judged on adjectives.