Air Purifier CADR: How Big a Room ₹10,990 Buys

₹10,990 is the lowest price listed for the Xiaomi Smart Air Purifier 4 Lite, at Croma, according to MySmartPrice's tracker (retrieved 5 October 2026). Buy it for the bedroom. Skip it for an open hall, which needs a second unit or a bigger machine. The deciding number is the air purifier CADR, and most of us read it backwards: as a promise about our room, when it measures the machine.

How much CADR do I need for my room?

You need a CADR, in cubic feet per minute, of at least two-thirds of your room's floor area in square feet, so a 150 sq ft room wants about 100 cfm on a normal day. That is the rule published by AHAM, the Association of Home Appliance Manufacturers, which runs the CADR certification itself. It lets you size a purifier with a tape measure instead of trusting the carton.

Indian listings complicate it. Xiaomi quotes the 4 Lite's output in cubic metres per hour, while AHAM's rule works in cubic feet per minute, and one cfm equals 1.699 m³/h. So the 4 Lite's rated output converts to about 212 cfm. Run that through the normal rule and it suits rooms up to roughly 318 sq ft: a bedroom with ease, a typical living room just about. That conversion is this article's own arithmetic, not a figure Xiaomi prints.

AHAM is stricter for smoke. In a wildfire-grade week it wants smoke CADR to equal the full floor area, not a share of it, which pulls the 4 Lite's comfortable limit down to about 212 sq ft. The usual advice is to buy for the average day. That advice is wrong for North India. October to January is when the machine earns its price, so size it for those weeks.

Four numbers settle the purchase: Xiaomi's spec as MySmartPrice lists it, the GST share in the sticker, the coverage AHAM's smoke rule removes, and the wait since Bar & Bench reported in December 2025 that the Delhi High Court had asked the GST Council to meet "at the earliest" on purifier tax. The running-cost thinking behind our 3-star vs 5-star refrigerator payback by state tariff applies here, as does the festive urge to stretch a budget with credit on UPI and how it is changing Indian spending.

The smoke figure is the one to plan around. A living room the 4 Lite handles in October can fall short on the November mornings you will judge the purchase by.

So which rooms does one unit actually cover?

Air purifier CADR for a bedroom versus a hall

One Xiaomi 4 Lite covers a 120 sq ft bedroom in any week, with headroom to spare, but a 350 sq ft open hall needs two units or a single, much larger machine. Needs come from AHAM; margins are this article's arithmetic. Power assumes 33 W, ten hours a day for 120 days, about 40 units at ₹8 a unit; tariffs vary by state.

The bedroom clears every row; the hall fails both coverage rows, and a second unit helps only if its doorways can be closed.

Is GST on air purifiers 5% or 18% right now?

Assume 18%, the rate the Delhi High Court case was fought over, unless the GST Council's proposed cut to 5% has since been notified; we found reports of the proposal but no notification. PTI reported on 3 January 2026 that the Council was weighing that rate. At 5%, this price would fall by roughly ₹1,200, which sounds like a reason to wait. It isn't, or not yet.

Six days later the Centre told the court that classing purifiers as medical devices cannot decide the GST rate. My read, and it is opinion: any cut will come through a Council notification, and festive sales will not wait for it. If a Diwali listing claims a big markdown, check the 30-day prior price that India's e-commerce amendment rules on dark patterns and prior prices require.

The other traps are physical.

• AHAM's rule assumes 8 ft ceilings. At 10 ft a room holds a quarter more air, so treat the 4 Lite as roughly a 254 sq ft machine on a normal day.
• The filter, not electricity, is the recurring cost; we could not confirm its price, so check mi.com.
• Every figure assumes a closed room.
• A carton's "up to" coverage is not an AHAM figure.

So decide this week. If your bedroom passes all four checks, buy the 4 Lite at the lowest price you find; if it is the hall you want cleaned, price two units against one larger machine before the smog arrives.