Air Purifier Size Guide Cadr Room India

Why Most Purifiers Are Undersized

Undersizing is the single most common reason people conclude air purifiers do not work. The
unit runs, the filter is genuine, and the room stays polluted — because the machine cannot move
enough of that room’s air per hour to make a difference.

The confusion comes from a number that is technically honest and practically misleading:
coverage area. Manufacturers are entitled to calculate it at a modest
air-change rate, so a unit sold for “up to 400 sq ft” may be rated at roughly two air changes an
hour. That is fine for a mild day and inadequate for a severe one.

CADR — The Only Number That Matters

Clean Air Delivery Rate is the volume of properly filtered air a unit
delivers per hour, normally quoted in cubic metres per hour (m³/h). Some brands quote cubic feet
per minute (CFM) instead — multiply CFM by about 1.7 to get m³/h.

What makes CADR the right number is that it combines the two things that actually determine
performance: how well the filter cleans and how much air the fan moves
through it. An H14 filter behind an underpowered fan produces a lower CADR than an H13
behind a good one — and protects less room.

A specification sheet that lists five technologies but no CADR figure is describing a product
that would rather you did not compare it.

ACH — How Many Times Per Hour

Air changes per hour is how many times the unit can process the room’s entire volume in an
hour. It converts CADR into something meaningful for your specific room.

Air changes/hour	Appropriate for
2 ACH	General comfort on a moderate day. What most coverage claims assume
3 ACH	Reasonable everyday target in a city
4–5 ACH	Severe pollution season, or anyone with asthma or allergies
6+ ACH	Rarely necessary at home; noise usually becomes the limit first

Working Out What You Need

Three steps, and the arithmetic is simple enough to do on a phone in a showroom.

1. Room volume in cubic metres.
   Floor area in sq ft × ceiling height in ft ÷ 35.3
   (A 12 × 12 ft room with a 10 ft ceiling: 144 × 10 ÷ 35.3 ≈ 41 m³)
2. Multiply by the air changes you want.
   41 m³ × 5 ACH ≈ 205 m³/h
3. That is your target CADR. Buy at or above it.

Measure your actual ceiling. Indian ceilings are commonly 10 ft but frequently higher in
older construction, and a 12 ft ceiling adds 20% to the volume — enough to move you a model up.

The Table — Common Indian Room Sizes

All figures assume a 10 ft ceiling. Add 20% for a 12 ft ceiling.

Room	Floor area	Volume	CADR @ 3 ACH	CADR @ 5 ACH
Small bedroom / nursery	10 × 10 ft	~28 m³	85 m³/h	140 m³/h
Standard bedroom	12 × 12 ft	~41 m³	125 m³/h	205 m³/h
Large bedroom	14 × 16 ft	~64 m³	190 m³/h	320 m³/h
200 sq ft room	~14 × 14 ft	~57 m³	170 m³/h	285 m³/h
Living room	16 × 20 ft	~91 m³	275 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 that few domestic units
reach, and buying one unit for a large open plan is usually a disappointment. Two smaller units
placed apart generally outperform one large one — and give you a fallback when one is being
serviced.

Translating the Box’s Claim

You will rarely find a CADR figure on the front of the packaging; you will always find a
coverage area. Two ways to convert:

• Halve it. Treat “up to 400 sq ft” as roughly 200 sq ft of genuine
  protection in a polluted winter. Crude, and reliable.
• Find the real number. CADR is usually in the technical specifications on
  the manufacturer’s website even when it is absent from the box. If you cannot find it anywhere,
  that absence is itself information.

Where Filter Grade Fits In

Filter grade matters, and it matters less than sizing.

Under EN 1822, H13 captures at least 99.95% of particles at the
most-penetrating size (around 0.3 microns) and H14 at least 99.995%. Both are
genuinely high. The phrases to distrust are “HEPA-type”, “HEPA-style” and
“99% HEPA” — none of which are held to the standard.

Given a choice between H14 with a low CADR and H13 with a high CADR for your room size,
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.

Five Sizing Mistakes

• Trusting the coverage claim. It assumes an air-change rate that a severe
  day does not respect.
• Ignoring ceiling height. Volume, not floor area, is what gets cleaned.
• Buying one unit for an open plan. The required CADR is usually beyond a
  single domestic machine.
• Sizing for maximum speed you will never use. If the top setting is too
  loud for the room, the real CADR is whatever the usable speed delivers.
• Forgetting the door. Every calculation here assumes a closed room. Open
  it and the volume being cleaned is the whole flat.

Frequently Asked Questions

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 x 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.

What CADR do I need for a 200 sq ft room?

A 200 sq ft room with a 10 ft ceiling is roughly 57 cubic metres. At three air changes per hour that needs a CADR around 170 m³/h; at five air changes, around 285 m³/h. For a genuine pollution season, size for five rather than three.

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. 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.

Why does my air purifier say 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 Delhi winter. A practical correction is to halve the printed area — if the result still covers your room, the sizing is genuinely adequate.