What Size Air Purifier for Allergies? CADR, ACH and the Box-Size Trap
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If you buy an air purifier for allergies, the size matters more than the brand. Allergies need 4 to 5 air changes per hour (ACH) — the purifier must clean the room’s full volume of air four or five times every hour. The number to shop by is CADR, and the formula is simple: room volume in cubic feet × ACH ÷ 60. A 12 × 10 ft bedroom with 8 ft ceilings is 960 cubic feet, so for allergies it needs about 80 CADR. Most purifiers sold for that room are far weaker than that — because the room size printed on the box is measured at roughly 2 ACH, which is too weak for allergy relief.
Work out what CADR do I need for my room size with the free calculator.
What CADR do I need for my room size?
CADR (Clean Air Delivery Rate) is the cubic feet of clean air a purifier delivers per minute, tested separately for smoke, dust and pollen. Always shop by the lowest of the three — and in practice that is usually the smoke CADR, because smoke particles are the smallest and hardest to catch. If a listing only quotes one CADR number, assume it is the flattering one and look for the full set before you buy.
The formula is: length × width × ceiling height × ACH ÷ 60 = CADR needed. The ACH target depends on what you are fighting:
- Allergies: 4–5 ACH
- Pets: about 4 ACH
- Everyday dust: 2–3 ACH
- Smoke: 5 ACH or more
Ceiling height matters as much as floor area. A room with 9 ft ceilings holds about 12% more air than the same room with 8 ft ceilings, so it needs about 12% more CADR. Run your room through the air purifier CADR calculator — enter your exact length, width and ceiling height and it does the maths for your situation.
Why is the room size on the box misleading?
This is the box-size trap, and it catches almost everyone. A manufacturer can claim a purifier “covers up to 1,700 sq ft” while only cleaning that space once per hour — 1 ACH. At 1 ACH the air is barely being filtered, and for allergies it is effectively not being filtered at all.
Room-size claims are usually based on about 2 ACH. That is fine for light dust in a room nobody sneezes in. For allergies you need 4–5 ACH, which means the room a purifier genuinely serves for allergy relief is roughly half the size printed on the box — sometimes less. A unit rated for “up to 400 sq ft” at 2 ACH only covers about 200 sq ft at 4 ACH.
The fix is to ignore the box and read the CADR instead. Divide the smoke CADR by 1.5 and you get roughly the room size it serves at 5 ACH with 8 ft ceilings. If the smoke CADR is 150, the purifier genuinely covers about 225 sq ft for allergies — whatever the box says.
What CADR do I need for a bedroom, a living room, and an open-plan space?
Here are the worked numbers, all at 5 ACH for allergies with standard 8 ft ceilings unless stated:
| Room | Size | Volume | CADR needed (allergies, 5 ACH) |
|---|---|---|---|
| Small bedroom | 12 × 10 ft | 960 cu ft | 80 |
| 200 sq ft room | 200 sq ft | 1,600 cu ft | 133 |
| Living room | 20 × 15 ft | 2,400 cu ft | 200 |
| Open-plan space | 25 × 20 ft, 9 ft ceilings | 4,500 cu ft | 375 (for smoke) |
Three bands fall out of this. Under 150 CADR covers small rooms up to about 200 sq ft. From 150 to 300 CADR covers a normal living room. Over 300 CADR is large-room territory for open-plan spaces. A 200 sq ft room needs about 107 CADR at 4 ACH (pets) and about 133 CADR at 5 ACH (allergies) — both inside the small-room band, which is why a modest purifier is genuinely enough for a bedroom, while the same unit would be lost in a living room.
What if I have pets as well as allergies?
Add 1 ACH on top of your base number. If allergies alone call for 5 ACH, a home with pets in it calls for the same 5 ACH — but if you were sizing for dust at 3 ACH, pets push you to 4. Pet hair itself is caught by the washable pre-filter, while dander — the actual allergen — floats in the air and needs the HEPA filter and enough air changes to reach it.
That is the only sizing change pets make. The picks themselves are a separate question, and our pet guide covers them in full: best air purifier for pets — 5 picks that tame hair, dander and odour. When comparing pet models, favour one with a washable pre-filter, because it catches the hair before it clogs the main filter.
Should I buy one big purifier or two smaller ones?
For one enclosed room, one correctly sized unit is almost always better: it is cheaper to run, quieter, and you only replace one set of filters. The exception is a long or L-shaped open-plan space, where a single unit at one end leaves the far end barely cleaned. Air does not mix evenly across 25 feet of open plan.
In that case two smaller units at opposite ends beat one big unit in the middle — each one only has to clean its own half, and you can run the nearer one on low at night. Two units also give you a spare when one filter is being changed. The maths is the same either way: split the volume in half and size each unit for its half at your target ACH.
Which purifiers actually hit these CADR numbers?
Two examples show how the numbers map to real models, with prices checked 9 November 2026:
- Levoit Core 400S-P — AHAM-verified smoke CADR of 231 CFM, which covers roughly 350 sq ft at close to 5 ACH. That is a living-room purifier, not a bedroom one, despite its compact size. $189.97 on Amazon (checked 9 November 2026). Check price on Amazon
- Coway Airmega 400 — AHAM-verified smoke CADR of 328 CFM, covering roughly 500 sq ft at close to 5 ACH, which is open-plan territory. $629.99 on Amazon (checked 9 November 2026); street prices are often lower, around $400–$450. Check price on Amazon
Notice what neither listing leads with: the box room size. The Levoit’s box talks about far larger rooms, because those figures assume a single air change per hour. Judge both by their smoke CADR against your room’s volume, not by the headline coverage.
If you want more ways to size things around the home, browse the rest of our free tools — every one answers a calculation people actually search for.
Frequently asked questions
What CADR do I need for a 200 sq ft room?
A 200 sq ft room with 8 ft ceilings is 1,600 cubic feet. At 4 air changes per hour for pets that is about 107 CADR; at 5 for allergies about 133 CADR. That sits in the small-room band (under 150 CADR, up to about 200 sq ft). Enter your exact room size in the calculator above for your number.
What CADR do I need for a large living room?
Use the large-room band: CADR over 300, for open-plan living spaces. A 25 × 20 ft space with 9 ft ceilings, for smoke at 5 air changes, needs 375 CADR minimum. A 20 × 15 ft living room with 8 ft ceilings, for allergies at 5 air changes, needs 200 CADR. Enter your room’s length, width and ceiling height in the calculator — ceiling height matters, because taller rooms hold more air.
What does ACH mean for allergies?
ACH is air changes per hour — how many times a purifier filters the room’s full volume of air in one hour. For allergies you need 4 to 5 ACH, because pollen and dander settle and re-circulate constantly; at 2 ACH the air is only cleaned twice an hour, which is not enough to keep allergen levels down while you are in the room.
Should I shop by smoke CADR or pollen CADR?
Shop by smoke CADR. It is almost always the lowest of the three CADR numbers, because smoke particles are the smallest and hardest to catch. A purifier that manages a high smoke CADR will handle pollen easily; the reverse is not true. If a listing only shows one CADR figure, check whether it is smoke before trusting it.
Is a higher CADR always better?
No. A purifier with far more CADR than your room needs costs more to buy, uses more power, and is louder on high — and on low it may be no better than a correctly sized unit. Size to your room and your ACH target, then spend the difference on a quieter model or a spare filter.
