What Air Purifier CADR Do You Actually Need? (India, 2026)
The number that decides whether an air purifier can actually clean your room is its CADR— Clean Air Delivery Rate — not the “covers up to 400 sq ft” on the box. That coverage figure is often calculated at just one or two air changes an hour, a background trickle. Here is the honest version, sized to a real 5 air changes per hour, plus a calculator for your own room.
Size an air purifier by air changes per hour (ACH), not the box's “coverage”. Aim for about 5 ACH for general use (6 for allergies, asthma or heavy Delhi-winter pollution). The formula is simple: required CADR (m³/h) = floor area (m²) × ceiling height (m) × ACH. As a shortcut at a 10 ft ceiling and 5 ACH, a purifier honestly covers about 0.7 sq ft per 1 m³/h of CADR — so a 300 m³/h unit truly suits a ~210 sq ft room, not the 400+ it's often sold for.
Always read the manufacturer's smoke CADR — the smallest-particle score — for PM2.5. 1 CFM = 1.699 m³/h.
CADR Calculator — What You Actually Need
Work out the honest CADR for your room, or check whether a purifier's advertised coverage holds up. Sized to a real 5 air changes per hour— not the 1–2 changes many “coverage” claims quietly assume.
For a 150 sq ft room at 5 ACH, look for a purifier with a CADR of at least
210 m³/h
(≈ 125 CFM) — the smoke/PM2.5 CADR, the smallest-particle score
Estimate only. Formula: required CADR (m³/h) = floor area (m²) × ceiling height (m) × ACH. Sized to 5ACH (CDC clean-air target; AHAM independently certifies 4.8 ACH at a unit's suggested room size). Use the manufacturer's published smoke CADR — the smallest-particle score — for PM2.5. Your result depends on ceiling height, door/window leakage and how the room is used.
CADR → Honest Room Coverage
The room a purifier honestlycovers at 5 air changes per hour and a 10 ft ceiling — compare this against the sq-ft on the box. Match the manufacturer's smoke CADR to the row at or above your room size.
| CADR (m³/h) | CADR (CFM) | Honest coverage (sq ft) @ 5 ACH |
|---|---|---|
| 100 | 59 | 71 |
| 150 | 88 | 106 |
| 200 | 118 | 141 |
| 250 | 147 | 177 |
| 300 | 177 | 212 |
| 350 | 206 | 247 |
| 400 | 235 | 283 |
| 500 | 294 | 353 |
| 600 | 353 | 424 |
Coverage = CADR(m³/h) ÷ (ceiling height in m × ACH), converted to sq ft. At a taller ceiling or a higher ACH goal, coverage is lower — use the calculator for your exact case.
Use this data
The CADR → honest-coverage table as a CSV. Free to reuse with attribution (CC BY 4.0).
How to cite: PakkaPick, “Air Purifier CADR for Room Size (India 2026),” pakkapick.in/air-purifier-cadr-for-room-size-india-2026 — CC BY 4.0
Why the “coverage” on the box is usually too big
A “covers up to X sq ft” claim is only as honest as the number of air changes behind it — and that number is almost never printed. Because coverage area is inverselyproportional to air changes, the same purifier looks twice as large on paper if it's rated at ~2.5 air changes an hour instead of a health-relevant 5. So a unit sold “for 400 sq ft” may only manage a couple of slow air changes there.
AHAM's own consumer rule of thumb is the two-thirds rule: a purifier's smokeCADR in CFM should be at least two-thirds of the room area in sq ft — which works out to roughly 5 air changes an hour at a standard 8 ft ceiling. AHAM adds that for ceilings above 8 ft you need a unit rated for a larger room. This page sizes to 5 ACH and a taller, more realistic 10 ft ceiling on purpose — it's the stricter, more honest end.
Why this matters more in India
The WHO's 2021 guideline for PM2.5 is 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours. On India's CPCB air-quality index, 24-hour PM2.5 is rated Good (0–30), Satisfactory (31–60), Moderate (61–90), Poor (91–120), Very Poor (121–250) and Severe (250+). Delhi and much of North India sit in the Very Poor to Severe bands through the winter — many times the WHO limit — with fresh polluted air leaking in continuously. That is exactly when an honestly-sized purifier (a real 5–6 air changes an hour) beats one rated for a bigger room on paper but delivering a trickle in practice.
Method & sources
Everything here is computed from one formula — required CADR (m³/h) = floor area (m²) × ceiling height (m) × air changes per hour — with unit conversions 1 CFM = 1.699 m³/h and 1 m² = 10.764 sq ft. No product specifications are used or invented; you supply the CADR the manufacturer publishes. The 5-ACH baseline follows the US CDC clean-air target and AHAM's certified 4.8-ACH suggested-room basis.
- AHAM Verifide — Air Filtration Standards (CADR definition, the 2/3 rule, >8 ft ceiling guidance)
- AHAM Verifide — Air Changes per Hour FAQ (certified 4.8 ACH basis)
- US EPA — Guide to Air Cleaners in the Home (CADR sizing, ~65 CFM per 100 sq ft)
- US CDC — Ventilation in Buildings (5 air changes/hour clean-air target)
- WHO — Global Air Quality Guidelines 2021 (PM2.5: 5 µg/m³ annual, 15 µg/m³ 24-hour)
- CPCB (Govt of India) — National Air Quality Index
Frequently Asked Questions
What is CADR on an air purifier?
CADR (Clean Air Delivery Rate) is the volume of filtered air a purifier delivers, measured by the Association of Home Appliance Manufacturers (AHAM) under the ANSI/AHAM AC-1 test. Every certified unit gets three scores — for smoke (the smallest particles, which matters most for PM2.5), dust, and pollen. Indian and European brands usually print it in cubic metres per hour (m³/h); US labels use cubic feet per minute (CFM). 1 CFM = 1.699 m³/h.
What CADR do I need for my room in India?
Size it to air changes per hour (ACH) — how many times an hour the purifier cleans a room-full of air. A good general target is 5 ACH (the US CDC clean-air target; AHAM independently certifies units to 4.8 ACH at their suggested room size). The formula is: required CADR (m³/h) = floor area (m²) × ceiling height (m) × ACH. For a 150 sq ft room with a 10 ft ceiling at 5 ACH, that is about 212 m³/h. Use the calculator above for your room.
Why is my air purifier's advertised 'coverage area' often misleading?
A coverage figure is only as honest as the air-change rate behind it — and that rate is rarely printed. Because coverage area is inversely proportional to air changes, the same purifier looks twice as large on paper if it is rated at ~2.5 air changes an hour instead of a health-relevant 5. So a unit sold "for 400 sq ft" may deliver only a slow couple of air changes in that room — a background trickle, not enough for allergies, asthma or a Delhi-winter PM2.5 load. AHAM's own rule of thumb is that a purifier's smoke CADR (in CFM) should be at least two-thirds of the room area in sq ft.
How many air changes per hour do I need for Delhi winter?
When outdoor air sits in the CPCB "Very Poor" or "Severe" band — routine for Delhi/NCR in November–January, with PM2.5 many times the WHO 24-hour guideline of 15 µg/m³ — aim higher than the 5 ACH general baseline, around 5–6+ ACH, so the purifier keeps clearing the PM2.5 that leaks in continuously. Particle removal follows exponential decay: about 2.3 air changes clear ~90% of a fresh load, and ~4.6 clear ~99%, so more air changes hold a lower steady-state level.
Does a higher ceiling need more CADR?
Yes — CADR scales with room volume, not just floor area. A 10 ft ceiling needs 25% more CADR than an 8 ft ceiling for the same floor area and air-change target. AHAM says explicitly that for ceilings above 8 ft you should choose a unit rated for a larger room. The calculator above lets you set your real ceiling height.
How do I convert air-purifier CADR between m³/h and CFM?
1 CFM = 1.699 m³/h. So to go from CFM to m³/h, multiply by 1.699; to go from m³/h to CFM, divide by 1.699. A 250 m³/h purifier is about 147 CFM.