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Buying guide

Air Purifier Room Size Calculator: The Simple CADR Rule

Before comparing brands, calculate the minimum CADR your room needs. This quick rule prevents the most common air purifier mistake: buying a model that is too small for the space.

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The simple formula

For a normal 8-foot ceiling, start with this:

Room square footage × 0.67 = minimum smoke CADR

That means:

Room sizeMinimum smoke CADR to start with
100 sq ft~67 cfm
150 sq ft~101 cfm
200 sq ft~134 cfm
300 sq ft~201 cfm
400 sq ft~268 cfm
600 sq ft~402 cfm

This is a baseline, not a perfect answer. It tells you whether a purifier is in the right range before you compare noise, filters, smart features, or price.

Adjust for ceiling height

If your ceiling is higher than 8 feet, adjust the room size first:

Adjusted room size = square footage × ceiling height ÷ 8

Example: a 200 sq ft room with a 10-foot ceiling behaves more like a 250 sq ft room. So the baseline CADR becomes about 168 cfm, not 134 cfm.

When to add more headroom

Choose a stronger purifier if:

  • You have wildfire smoke, cooking smoke, or frequent odors.
  • The room is open to other rooms.
  • You have pets.
  • You are sensitive to noise and will run the purifier on lower speeds.
  • The purifier will sit far from the problem area.

A purifier with extra headroom can clean more effectively on a lower, quieter setting.

Examples

12 x 12 bedroom

A 12 x 12 bedroom is 144 sq ft. The baseline smoke CADR is about 96 cfm. A real bedroom purifier should exceed this comfortably so it can run quietly.

12 x 14 bedroom

A 12 x 14 bedroom is 168 sq ft. The baseline is about 112 cfm. This is where a full-size bedroom purifier makes more sense than a mini purifier.

20 x 20 living room

A 20 x 20 living room is 400 sq ft. The baseline is about 268 cfm. For smoke, pets, open layout, or quiet operation, you may want more.

PickGrade recommendation

Once you know your room size, use the quiz to choose between a bedroom model, mid-size automatic purifier, large-room purifier, or tiny-space option. Room size narrows the field fast; noise, pets, smoke, and budget decide the final pick.

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Frequently asked

How do I calculate the right air purifier size?

For an 8-foot ceiling, multiply room square footage by 0.67 to estimate the minimum smoke CADR. Then add headroom for smoke, pets, high ceilings, open layouts, or quiet operation.

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

For an 8-foot ceiling, start around 134 cfm smoke CADR. A stronger purifier may be better if you plan to run it quietly or have pets, smoke, or high ceilings.

What CADR do I need for a 400 sq ft room?

Start around 268 cfm smoke CADR for a normal 8-foot ceiling. For smoke or open layouts, choose more headroom.

Can an air purifier be too big for a room?

Usually not in a practical sense. A larger purifier can often run at a lower, quieter speed. The trade-offs are size, price, and filter cost.

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