Air Purifier CADR by Room Size: How Much CADR Do You Actually Need? (India Guide)

Short answer: multiply your room's floor area in square feet by 0.67. That gives you the minimum smoke CADR in CFM for a room with an 8-foot ceiling. Indian brands usually quote CADR in m³/h, so multiply the CFM figure by 1.7. The shortcut is room sq ft × 1.14 = minimum m³/h:

• A 150 sq ft bedroom needs about 170 m³/h.
• A 300 sq ft living room needs about 340 m³/h.

For Delhi-NCR winter smog, buy one size bigger than the minimum so the purifier can run quietly on a low setting.

The "covers up to 400 sq ft" line on an Indian air purifier box is not a standard. Each brand picks its own number. CADR (Clean Air Delivery Rate) is the one figure you can compare across brands, but only once you know how to read it. This guide gives you the formula, a room-size table, the ceiling-height fix and a five-point checklist, then points to purifiers currently on deal whose CADR you can check against your room.

What CADR means, in one paragraph

CADR is the volume of already-cleaned air a purifier delivers per unit of time. It is tested separately for three particle types:

• smoke, the smallest particles, closest to PM2.5;
• dust;
• pollen, the largest particles.

Smoke CADR is always the lowest of the three and the most relevant to Indian city pollution, so it is the one to size by. If a listing quotes a single CADR without saying which particle, assume it is the most flattering number and treat the sizing as optimistic.

The 2/3 rule: the formula behind every sizing chart

The sizing rule used in AHAM's guidance (AHAM is the US body that runs the CADR test) is:

Minimum smoke CADR (CFM) = room area (sq ft) × 2/3

This assumes an 8-foot ceiling (about 2.4 m), which matches most Indian flats. A purifier that meets the rule cleans the room's full air volume about 5 times an hour (5 air changes per hour, or ACH). That is the range usually recommended for allergy and smoke relief.

To convert for Indian spec sheets:

• 1 CFM ≈ 1.7 m³/h
• So minimum m³/h ≈ room sq ft × 1.14

Room-size table (8 ft ceiling)

Room	Floor area	Minimum smoke CADR (CFM)	Minimum CADR (m³/h)	Smog-season target (m³/h, ~1.5×)
Small study / kids' room	100 sq ft	67	114	170
Standard bedroom	150 sq ft	100	171	255
Master bedroom	200 sq ft	134	228	340
Living room (2BHK)	300 sq ft	201	342	510
Large hall / open plan	400 sq ft	268	456	685

The last column is our recommendation, not a test standard. On the high-AQI days of November and December, a purifier running at full speed is loud enough that most people turn it down at night. Buying about 1.5× the minimum CADR means the "sleep" or low setting still does the job.

Worked example: a 12 × 14 ft bedroom

1. Area: 12 × 14 = 168 sq ft.
2. Minimum CFM: 168 × 0.67 = 113 CFM.
3. Convert to m³/h: 113 × 1.7 ≈ 192 m³/h.
4. Smog-season target: 192 × 1.5 ≈ 290 m³/h.

So a purifier rated around 170 m³/h is slightly under-sized for this room. One rated 250-300 m³/h is the comfortable pick if you live in Delhi, Lucknow, Kanpur or any city where winter AQI regularly crosses "very poor".

Ceiling height changes the answer

The 2/3 rule assumes 8 ft. Taller rooms hold more air, so you need proportionally more CADR:

Ceiling height	Multiply minimum CADR by
8 ft (2.4 m)	1.00
9 ft (2.7 m)	1.13
10 ft (3.0 m)	1.25
12 ft (3.7 m), old bungalows, duplex halls	1.50

Older independent houses and duplex living rooms often have 10-12 ft ceilings. Sizing them by floor area alone under-buys by 25-50%.

Why "covers 400 sq ft" on the box is not the same thing

Indian product pages often state coverage using a looser rule of thumb, roughly sq ft ≈ CADR (m³/h) × 1.5. Under that rule, a 200 m³/h unit "covers" 300 sq ft. By the 2/3 rule, the same unit covers only about 175 sq ft at 5 air changes per hour. The coverage claim is not false. It just assumes the air in your room is cleaned about 3 times an hour, not 5, which is too slow when pollution keeps leaking in through window gaps and every time a door opens.

What to do with this:

• Ignore the coverage claim. Read the CADR number in the spec table or the manual.
• Check which CADR it is. "Smoke" or "PM2.5" CADR is what you want. A single unlabelled CADR may be the higher pollen or dust figure.
• Prefer third-party tested numbers. Some brands publish AHAM-verified or lab-certified CADR. Others publish an in-house figure. When two units have similar CADR, the tested one is the safer bet.

Real purifiers on deal, checked against the table

These are live deals on RichDeals at the time of writing. Prices move daily, so open each deal page for the current price. The CADR figures below are the ones stated in the product listing.

Purifier	Stated CADR	Room it fits (2/3 rule, 8 ft)	Deal price at writing
LINSO FLO UVC air purifier	172 m³/h	~150 sq ft bedroom	₹4,999
LINSO STORM UV-C + HEPA H13 (Wi-Fi)	457 m³/h	~400 sq ft hall	₹18,999

The LINSO FLO is a neat example of a unit sized honestly for one bedroom. The STORM has enough CADR for a 2BHK living room even in smog season, as long as the room is under about 300 sq ft.

For the purifiers below, check the CADR on the product page against the table above before you buy:

• Eureka Forbes Air Purifier 150 (HEPA H13), ₹4,999 at writing. It is marketed for 200 sq ft.
• pTron EaseMate H1, ₹2,999 at writing. It is a budget pick for a small study or kids' room.
• Havells Studio Breathe AP310 with AQI display, ₹8,451 at writing.
• Dyson Big+Quiet HEPA H13. It is marketed for large spaces up to 1,100 sq ft.

Already own a purifier? CADR drops as the HEPA filter clogs, so a filter that is overdue is the cheapest "upgrade". The Philips FY3200/30 replacement filter 2-pack and the TROO filter for Xiaomi Mi purifiers are both on deal.

For a full shortlist by budget, see our best air purifiers in India 2026 guide. If your main problem is dust or allergies rather than smog, read do air purifiers help with dust allergy. For every live home-appliance price drop, check today's offers.

Five-point CADR checklist before you pay

1. Measure the room. Length × width in feet. Note the ceiling height if it is above 8 ft.
2. Do the maths. Room sq ft × 1.14 = minimum m³/h. Then apply the ceiling multiplier.
3. Size up for smog. In North Indian winters, aim for about 1.5× the minimum so the low setting is enough at night.
4. Read the smoke or PM2.5 CADR, not the coverage claim and not the pollen figure.
5. Price the filter. A purifier with a cheap, easy-to-find replacement filter keeps its CADR. One with a rare, expensive filter often runs clogged.