Is a bigger air purifier always better?

Practically, yes, because you can run an oversized unit on its quiet setting and still get the airflow you need. The cost is price and floor space, not performance.

What CADR do I need for a bedroom?

For a typical 300 sq ft bedroom, around 180 CFM for four air changes an hour — roughly double what the box's coverage claim implies.

Do I need one air purifier per room?

Per room you actually use, yes. A closed door stops airflow, so a single unit does not serve a whole house regardless of its rating.

How many air changes per hour do I need for allergies?

Four to five. Two is adequate for ordinary dust but too slow to keep pollen or dander concentrations down during a flare-up.

Where should I place an air purifier?

Near the source of the problem, with clear space around the intake and outlet, and in the room you spend the most time in. Corners and behind furniture waste most of its capacity.

Should I run it all the time?

Yes — continuously on a low setting beats intermittent bursts on high, because particle levels rebuild within an hour or two of switching off.

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What size air purifier do I need for my room?

One number decides this, and it isn't the square footage on the box

Published by AffiliateHubPro Last reviewed August 11, 2026

Multiply your room's floor area in square feet by 0.6 to get the CADR you need for four air changes an hour — the rate that actually matters for allergies and smoke. A 300 sq ft bedroom therefore wants a CADR of about 180, not the '500 sq ft' claim on a box, because manufacturers quote coverage at a much gentler two changes an hour. When in doubt, buy larger than you need and run it on low: an oversized purifier on a quiet setting outperforms a correctly-sized one you turn off because it's too loud.

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The arithmetic, in one place

CADR — clean air delivery rate — is the volume of filtered air a unit produces per minute or hour. Air changes per hour (ACH) is how many times that volume replaces the air in your room. Two ACH is enough for general dust. Four to five is the target if anyone has allergies or asthma, or if you're dealing with smoke or wildfire season.

The rule of thumb for standard eight-foot ceilings: needed CADR (in cubic feet per minute) is roughly floor area × 0.6 for four ACH. For higher ceilings, scale it by the actual ceiling height over eight. Metric CADR figures are usually quoted in m³/h; divide by 1.7 to compare against a CFM number.

The catch is that the coverage figure on the packaging is nearly always calculated at two ACH and on the highest, loudest fan setting. That is why a box claiming 500 sq ft is honestly a 250 sq ft purifier for anyone with allergies, and a much smaller one still at a noise level you'd tolerate overnight.

Room	Floor area	CADR for 4 ACH	Typical box claim
Small bedroom / office	150 sq ft	~90 CFM	300 sq ft
Master bedroom	300 sq ft	~180 CFM	500-600 sq ft
Living room	500 sq ft	~300 CFM	1,000 sq ft
Open-plan kitchen/living	800 sq ft	~480 CFM	1,500 sq ft+

Why noise decides whether the purchase works

A purifier only cleans air while it is running, and the setting people actually leave on overnight is the lowest one. So the number that matters in practice is the CADR at the quiet setting, not the maximum — and manufacturers publish the maximum.

This inverts the usual buying advice. Deliberately buying a unit rated for a much larger room means its low, near-silent setting delivers the airflow a smaller unit would need its noisy setting to match. It costs more up front and less in irritation, and it is the single most common regret people report about their first purifier.

Bedrooms have a second constraint: light. Status LEDs bright enough to read by will get taped over. A model with a proper display-off mode is worth seeking out for a room you sleep in.

One big unit or several small ones

Air does not move freely between rooms, and a closed door effectively ends a purifier's coverage. A single large unit in a hallway does very little for the bedrooms off it. If the goal is sleeping and breathing better, the unit belongs in the room where you sleep, sized for that room only.

For an open-plan space, one correctly-sized unit beats two small ones, mostly because the fan needs enough throw to actually circulate air across the whole volume rather than politely filtering the corner it stands in.

Placement matters more than people expect: give it clearance on all sides, keep it off the floor against a wall if the intake is at the base, and put it near the source — the litter tray, the kitchen doorway, the open window during pollen season.

Common questions

Is a bigger air purifier always better?: Practically, yes, because you can run an oversized unit on its quiet setting and still get the airflow you need. The cost is price and floor space, not performance.
What CADR do I need for a bedroom?: For a typical 300 sq ft bedroom, around 180 CFM for four air changes an hour — roughly double what the box's coverage claim implies.
Do I need one air purifier per room?: Per room you actually use, yes. A closed door stops airflow, so a single unit does not serve a whole house regardless of its rating.
How many air changes per hour do I need for allergies?: Four to five. Two is adequate for ordinary dust but too slow to keep pollen or dander concentrations down during a flare-up.
Where should I place an air purifier?: Near the source of the problem, with clear space around the intake and outlet, and in the room you spend the most time in. Corners and behind furniture waste most of its capacity.
Should I run it all the time?: Yes — continuously on a low setting beats intermittent bursts on high, because particle levels rebuild within an hour or two of switching off.

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