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What Size Air Purifier Do You Need for Your Room?

An air purifier only helps if it is big enough for the room. Here is how to tell before you buy, and the mistake we made with ours.

By Franco N.·Updated September 26, 2026

It is easy to buy an air purifier that is too small for the room. It happened to us with the Levoit Core 300-P.

We bought one in April 2021 and put it in our bedroom. We found it was too small for the room, so we moved it to our home office, which is smaller.

So start with the room, and let the purifier come second.

How much clean air your room needs

  • 100 sq ft room: 65 cfm
  • 150 sq ft room: 97 cfm
  • 200 sq ft room: 130 cfm
  • 300 sq ft room: 194 cfm
  • 400 sq ft room: 259 cfm

The smallest smoke CADR for each room, from AHAM’s room-size formula. For allergies, a high ceiling or cleaner air than the minimum, size up. Then check the room size on the purifier’s AHAM seal.

How big is my room, really?

Get the tape measure out. Measure the length and the width of the floor, multiply them, and find your room in the chart above.

The number on your line is the smoke CADR, which is how much clean air a purifier puts out each minute. Buy one whose smoke CADR is at least that, or whose certified room size is at least as big as your room.

If your room lands between two lines, go up to the bigger one. Do the same if the ceiling is high, because a taller room has more air in it to clean.

If you are buying because someone in the house has allergies, I would go up a size as well.

The listing says it covers a huge room. Will it cover mine?

Go by the room size on the AHAM seal, and set the one in the listing title aside. Independent labs check the number on the seal, which is why I trust it over anything a listing says.

Our Core 300 is a good example. Its Amazon title says “Up to 1073 ft²”. AHAM certifies the same purifier for 222 square feet, a much smaller room than the title suggests.

The bigger number is not made up. It is the size of room the purifier can get through once an hour. The certified size is the room it can clean almost five times an hour, and that is the one I would buy by.

You will find it on the seal on the box, or by searching the model in AHAM’s online directory. If a room size in a listing has no seal behind it, treat it as a sales number.

Which purifier fits my bedroom, nursery or office?

Once you know how big your room is, pick the purifier whose room size is at least that big. If you are torn between the three AeraMax sizes, the AeraMax 90 vs 190 vs 290 comparison puts them side by side.

Fellowes AeraMax 90For a room up to 100 sq ft
Fellowes AeraMax 190For a room up to 200 sq ft
Levoit Core 300For a room up to 222 sq ft, certified by AHAM
Fellowes AeraMax 290For a room up to 300 sq ft

What about the living room?

Measure it the same way. If it comes out bigger than every room size on the cards above, buy a purifier certified for a room at least as big as your living room.

Buying small for the biggest room in the house is the same mistake we made in our bedroom.

What if the problem is smoke, dry air or the filters?

Smoke and smells

A HEPA filter catches dust and other particles, but it does nothing for smells or gases. For smoke and odors, you want activated carbon, a filter made to trap them.

All three AeraMax sizes come with a carbon filter alongside the HEPA one. The Levoit Core 300’s standard filter is made for dust and light odors, and for smoke Levoit sells a Toxin Absorber filter with added activated carbon. The AeraMax comparison and the Core 300’s filter section cover both.

Dry air

Indoors, aim for humidity between 30 and 50 percent. If what bothers you is dry air, you want a humidifier or an air washer instead.

Air washer vs humidifierWhat each one does, and which one you need
Boneco W200An air washer for a dry room, rated by Boneco up to 538 sq ft
Boneco H680A humidifier and purifier in one, and what it costs

Filters

Think about the filters before you buy, too. That was the part my wife did not like about ours. She kept having to buy new filters, because they were not washable.

Check what the filters cost and how often they need changing, so they do not come as a surprise. The AeraMax and Boneco replacement filters guide covers those two brands, and the Core 300 review covers Levoit’s.

Every air care page

Questions people ask

What does CADR mean on an air purifier?

It stands for clean air delivery rate, which is how much filtered air the purifier puts out each minute, in cubic feet. You will see separate numbers for smoke, dust and pollen, and the smoke one is what the room size comes from.

How do I work out the room size from the CADR?

Multiply the smoke CADR by 1.55. The Core 300’s smoke CADR of 143 comes out at 222 square feet, which is its certified room size.

Why does the Amazon listing say a much bigger room?

Take the Core 300. Levoit’s own page labels its big number “1 ACH”, which is the room it can clean once an hour. Go by the certified room size on the seal instead.

Does ceiling height change what size I need?

A taller room holds more air, so it needs a bigger purifier. AHAM’s room sizes, and the chart at the top of this page, assume an 8-foot ceiling. If yours is taller, put your real height into Harvard’s sizing tool.

How many air changes an hour should a purifier manage?

The certified room size assumes 4.8, and I would aim higher if you can. Harvard’s sizing tool calls 6 ideal. To get there, buy a purifier certified for a room at least a quarter bigger than yours.

Sources