Air purifier planner
Which CADR does your room need — and when do the filters go?
One multiplication decides whether a purifier works in your room; one calendar decides whether it keeps working. Both are in the guides on paper. Here they are live, for your room, your hours and your electricity bill.
Your room
20 m² × 2.5 m × 4 = 200 m³/h
CADR needed = floor area × ceiling height × air changes. The guides work with a 2.5 m ceiling; change it if yours is higher, rather than adjusting by eye.
CADR your room needs
200 m³/h
8 of the 8 machines sold in your market reach it. Pick one to plan its filters below.
| Machine | CADR | ACH here | Box claims |
|---|---|---|---|
| 500 | 10 ✓ | 60 m² · 3.3 ACH | |
| 470 | 9.4 ✓ | 85 m² · 2.2 ACH | |
| 400 | 8 ✓ | 48 m² · 3.3 ACH | |
| 310 | 6.2 ✓ | 40 m² · 3.1 ACH | |
| 303 | 6.1 ✓ | 33 m² · 3.7 ACH | |
| 300 | 6 ✓ | 36 m² · 3.3 ACH | |
| 270 | 5.4 ✓ | 28 m² · 3.9 ACH | |
| 240 | 4.8 ✓ | 30 m² · 3.2 ACH |
The last column is the coverage printed on the box and the air-change rate it implies at a 2.5 m ceiling — mostly about 3.3, below the 4 to 5 you want for smog. Size up from it.
The machine and its year
Xiaomi Smart Air Purifier 4
Gives this room 8 air changes an hour — at or above the 4 you asked for.
Filter calendar
- Vacuum the pre-filter
- 15 Oct 2026 then monthly
- Rinse the pre-filter, dry fully
- 15 Dec 2026 then quarterly
- Replace the carbon stage
- 15 Jun 2027 every 9 months
- Replace the HEPA element
- 15 Mar 2028 every 18 months
Intervals are the maker’s for continuous running, stretched for fewer hours a day but never past one and a half times — media ages and carbon re-releases regardless. Set these as reminders; the machine’s own indicator is usually a run-hour counter.
What 3 years cost
- Electricity a year
- ₹1,051 131.4 kWh
- Filters a year (band 1 of 3, placeholder)
- ₹2,000
- Purchase price
- ₹13,999
₹23,153to buy and run for 3 years
The filter line is the guide’s placeholder for the machine’s cost band, not a price — replace it with a quote before trusting the total. Electricity is watts × hours × your tariff, exactly.
Where the numbers come from
- CADR figures and coverage are the catalogue’s, verified against spec sheets. Air changes an hour = CADR ÷ (floor area × ceiling height); 4 to 5 is the target the CADR guide gives for smog and allergies, 2 to 3 for general freshening.
- Filter intervals are the ones the running-costs guide quotes for continuous running — HEPA 6 to 12 months, carbon 3 to 6. Fewer hours a day stretch them by 24 ÷ hours, capped at 1.5×. The pre-filter is washable and never replaced.
- Electricity is watts × hours × 365 ÷ 1000 × tariff. The defaults are the guide’s: 30 W, ₹8 per unit in India, ¥0.55 in China. Read your own rating plate and bill.
- The three-year total is purchase price plus three years of filters and electricity — the same sum the guide works on real rows, with the same placeholder filter figures per cost band.
Read: understanding CADR and room size · Read: air purifier running costs · Find an air purifier