What size air purifier do you need?
Undersizing is the most common reason people conclude air purifiers don’t work. The coverage area printed on the box is calculated at a low air-change rate — defensible marketing, and a poor guide to an Indian winter. Work from your room’s volume instead.
Length × width. A 12 × 12 ft room is 144.
Measure it. A 12 ft ceiling adds 20% to the volume.
Room volume 40.8 m³ × 5 air changes per hour. That is roughly 120 CFM if a brand quotes it that way.
Comfortably within the range of a single domestic unit. Check the CADR figure in the specifications rather than the coverage area on the box.
Enter the coverage area printed on the box (sq ft). Manufacturers calculate it at about 2 air changes an hour.
The number that matters
CADR — Clean Air Delivery Rate — is the volume of genuinely filtered air a unit delivers per hour. It combines filter efficiency and fan capacity into one figure, which is why it matters more than filter grade alone: an excellent filter behind a weak fan produces a low CADR and cannot protect a large room.
A specification sheet listing five technologies but no CADR figure is describing a product that would rather you didn’t compare it.
How many air changes you should aim for
- 2× — comfort — What most coverage claims on the box assume
- 3× — everyday — Reasonable year-round target in a city
- 5× — pollution season — Severe AQI, asthma or allergies, a nursery
Common Indian room sizes
Assuming a 10 ft ceiling. Add 20% for a 12 ft ceiling.
| Room | Size | Volume | CADR @ 3× | CADR @ 5× |
|---|---|---|---|---|
| Small bedroom / nursery | 10 × 10 ft | 28 m³ | 85 m³/h | 140 m³/h |
| Standard bedroom | 12 × 12 ft | 41 m³ | 120 m³/h | 205 m³/h |
| Large bedroom | 14 × 16 ft | 63 m³ | 190 m³/h | 315 m³/h |
| Study / home office | 10 × 12 ft | 34 m³ | 100 m³/h | 170 m³/h |
| Living room | 16 × 20 ft | 91 m³ | 270 m³/h | 455 m³/h |
| Open-plan living + kitchen | ~400 sq ft | 113 m³ | 340 m³/h | 565 m³/h |
The last row is worth pausing on. Open-plan spaces need CADR figures few domestic units reach — two smaller units placed apart generally outperform one large one, and leave you covered while one is being serviced.
Three things sizing can’t fix
- An open door. Every figure here assumes a closed room. Leave it open and the volume being cleaned is the whole flat.
- Noise at the speed you need. CADR is quoted at maximum fan speed; noise is usually quoted at minimum. A purifier too loud to run overnight gets switched off at bedtime — which is the eight hours it was bought for.
- A spent filter. Efficiency holds up as a filter loads, but airflow falls — and falling airflow means falling clean-air delivery. In a polluted winter, a filter rated for a year can be finished in a season.
Frequently asked
- What size air purifier do I need for my room?
- Work from room volume, not the coverage area on the box. Multiply floor area by ceiling height to get cubic metres, then multiply by the air changes per hour you want — five for meaningful protection in polluted conditions. A 12 × 12 ft room with a 10 ft ceiling is about 41 m³, so five air changes needs a CADR near 205 m³/h.
- What does CADR mean on an air purifier?
- Clean Air Delivery Rate — the volume of genuinely filtered air a unit delivers per hour, usually in cubic metres per hour. It combines filter efficiency and fan capacity into one figure, which is why it matters more than filter grade alone: an excellent filter behind a weak fan produces a low CADR and cannot protect a large room.
- Why does my air purifier claim it covers 400 sq ft?
- Because coverage claims are calculated at a low air-change rate, often around two per hour. That is a defensible marketing figure and a poor guide for a pollution season. A practical correction is to halve the printed area — if the result still covers your room, the sizing is genuinely adequate.
- How many air changes per hour do I need?
- Two to three is often quoted for general comfort. Four to five is the sensible target where outdoor pollution is severe, or where someone in the room has asthma or allergies, or for a nursery. The higher figure is what turns a purifier from a device that makes a modest difference into one that keeps a room meaningfully cleaner than outside.
- Does filter grade matter more than CADR?
- No. Given a choice between H14 with a low CADR and H13 with a high CADR for your room, take the H13. The difference between 99.95% and 99.995% on a single pass is trivial next to the difference between three air changes an hour and one. Look for a stated H13 or H14 grade — 'HEPA-type' and 'HEPA-style' are not the standard.
- Should I buy one large air purifier or two smaller ones?
- For an open-plan space, two smaller units placed apart usually outperform one large one. Air mixing is imperfect in a big room, two units cover it more evenly, and you keep filtration running while one is being serviced. For a single closed room, one correctly sized unit is simpler and quieter.