What Is a Good CADR Rating for Air Purifier (With Calculator)

On this page you’ll read information on how to size an air purifier when buying a new one. And we’ll discuss how to know the room size an existing air purifier will serve.

Plus, there is talk about the impact a HEPA filter will have on indoor air quality, definition of terms and other important related topics.

But first, let’s be sure we’re understanding the terms used.

What does CADR Mean on an Air Purifier?

Here’s the meaning of CADR and related acronyms.

CADR stands for Clean Air Delivery Rate. It is expressed in CFM, or cubic feet per minute.

For example, a CADR of 267 means that the unit will take in, filter, and discharge 267 cubic feet per minute.

ACH is air changes per hour. Discussed later on this page, ACH represents how many times in an hour all the air can be purified.

For example, if the CADR of an air purifier is 250 and you place it in a room that is 125 square feet, it is capable of purifying all the air two times per hour, and ACH of 2, since 125 x 2 = 250 square feet.

What is a Good CADR Rating for an Air Purifier?

Here is the quick answer. This topic is more comprehensively explained below for those who want full details.

The minimum air purifier CADR is determined by multiplying the room’s square feet by 2/3 (two-thirds or .667). For example, the ideal CADR for a 400 square foot area is determined by this equation: 400 x .667 = 267 minimum CADR rating. The ideal CADR might be higher if you want the air in the room purified more than once per hour, a topic addressed below.

Room size x 2/3 = Minimum CADR

For example, a 240 square foot room needs an air purifier with a minimum CADR of 160.

*CADR is expressed in CFM of airflow, that is, how many cubic feet per minute of air it will clean.

This calculation is given by AHAM, the Association of Home Appliance Manufacturers.

Do you already have an air purifier and want to know how many square feet it will cover?

CADR x 1.55 = Room size it will serve

For example, If you have an air purifier with a CADR of 200, it will serve a room of about 300 square feet.

200 cfm x 1.55 = 310 square feet

Tools to Determine the Right CADR Rating for an Air Purifier

Pick HVAC has developed two easy-to-use tools for you to answer the question – What size air purifier do I need?

CADR Calculator

Plug in your room size, and our Pick HVAC CADR Calculator shows the Minimum Required CADR if you want one air change per hour.

CADR Calculator

For example, what size air purifier for 1000 square feet?

Type 1000 into the calculator, and you’ll see that a space with 1000 square feet needs an air purifier with a clean air delivery rate of 667 cubic feet per minute, or CFM.

Example two: What’s the right air purifier for 2000 square feet?

Type in 2000, and the calculator gives the answer: 1,334 CFM.

How big is your room? Measure the length times the width of the room to determine its area.

For example, a bedroom that is 15 feet wide and 20 feet long is 300 square feet.

15 x 20 = 300

Is smoke CADR different?

No, but as we discuss later, choose an air purifier with a true HEPA filter. And if it has a smoke and odor trapping charcoal filter, that’s a great addition.

Clean Air Delivery Rate – CADR Table

If you have multiple rooms that you’re considering for an air purifier, then save time with this CADR Rating Table – It might be faster than typing a handful of numbers into our CADR Calculator.

What Is A Good CADR Rating for an Air Purifier?

Room Size (sq ft)Air Purifier's minimum CADR
200 sq ft134 cfm
300 sq ft200 cfm
400 sq ft267 cfm
500 sq ft334 cfm
600 sq ft400 cfm
700 sq ft467 cfm
800 sq ft534 cfm
1,000 sq ft667 cfm
1,200 sq ft800 cfm
1,500 sq ft1,000 cfm
1,800 sq ft1,200 cfm
2,000 sq ft1,334 cfm
3,000 sq ft2,000 cfm

Let’s use the same examples:* Following AHAM’s 2/3 rule: Room size (sq. ft.) x 2 / 3 = Air Purifier’s minimum CADR

Ex. 1: What size air purifier for 1000 square feet?

Find the number 1000 in the Room Size (sq ft) column on the left. Scan across to the Air Purifier Minimum CADR column on the right, and you’ll see that the 1000 square foot room needs an air purifier with a Clean Air Delivery Rate, or CADR, of at least 667 cubic feet per minute, or CFM.

Ex. 2: What’s the right air purifier for 2000 square feet?

One with a CADR of 1,334 CFM minimum. Once you’ve located 2000 in the left column, it is easy to see the 1334 CFM in the right column.

Buying Tip: If you are buying an air purifier for an extra large room, like an open floor plan living room and kitchen combination of 1000 square feet, then one air purifier can clean the air in the entire space.

But let’s take the same 1000 square feet and put it into a 2-story bungalow or apartment.

We’d recommend one air purifier for each floor to get the cleanest air. This is especially true if the space is heated by a mini split heat pump or boiler system – in other words, an HVAC system that doesn’t use forced air to circulate air throughout the entire home or apartment.

Example: My 2-story apartment is 1000 square feet. What’s the right air purifier for 1000 square feet apartments?

Our recommendation would be to size an air purifier for each floor. If both floors are 500 square feet, then you would have the cleanest air with two air purifiers with a 334 CADR minimum.

If the dwelling has 600 square feet downstairs and 400 square feet upstairs, then consider a unit with a CADR of 400 CFM for downstairs and one with a CADR of 267 for upstairs.

Air Changes per Hour with Air Purifier by Room Size

See the Table next, but here’s a brief introduction to Air Changes per Hour, or ACH.

CADR or Clean Air Delivery Rate is based on one complete change of air per hour, meaning all the air in the room is purified each hour.

CADR = 1 ACH or Air Changes per Hour

Key information: If you want more than 1 ACH – air change per hour – then you’ll need an air purifier with a higher CADR for the same size room.

Example: Let’s say you have an extra large room of 600 square feet. For 1 ACH, the CADR needs to be 400 CFM according to the chart above.

So – an air purifier with a CADR of 400 CFM will change the air in a 600 square foot room 1 time each hour.

But, if you want 2 ACH in that room, you’ll need an air purifier with twice the capacity.

An air purifier with a CADR of 400 will change the air:

1 time per hour in 600 square feet

2 times per hour in about 300 square feet

3 times per hour in about 200 square feet

4 times per hour in about 150 square feet

Air Changes per Hour Table

The above explanation should help make sense of this chart.

Here is the Table. There are more explanatory notes below it.

Air Purifier Room Size:

50 CFM78 sq ft39 sq ft26 sq ft20 sq ft
100 CFM155 sq ft78 sq ft52 sq ft39 sq ft
125 CFM194 sq ft97 sq ft65 sq ft49 sq ft
150 CFM233 sq ft116 sq ft78 sq ft59 sq ft
200 CFM310 sq ft155 sq ft104 sq ft78 sq ft
240 CFM372 sq ft186 sq ft124 sq ft93 sq ft
300 CFM465 sq ft233 sq ft155 sq ft117 sq ft
350 CFM543 sq ft271 sq ft181 sq ft136 sq ft
400 CFM620 sq ft310 sq ft207 sq ft155 sq ft

Notes:* CADR (cfm) x 1.55 = Room size in square feet;

1. What the Table shows is how much space air purifiers will serve based on CADR and how many air changes per hour, or ACH, you want.

2. Obviously, the more ACH, the cleaner the air. The filter type in the air purifier makes a difference too, whether it is true HEPA, HEPA-like or a cheaper air filter, and that is discussed later.

3. “I have an air purifier – how many square feet will it purify?” With this approach, start at the left column labeled CADR. Scan down to find the exact or nearest CADR to your air purifier. Then scan right to find out how much space it will serve based on 1-4 ACHs.

4. “I need to buy an air purifier.” OK. The first question is, “how many times do you want the air to change each hour?” If your air purifier has a HEPA filter, aka “true HEPA,” then 1 or 2 times should be sufficient unless the indoor air quality is really bad. Then maybe 3 times or 4 times would do the job. If it isn’t true HEPA, then 2-4 ACH is right under normal indoor air quality conditions.

So, find the number you want under Air Changes per Hour (ACH). Then scan down to find the room size or closest room size. Finally, scan to the left to find the CADR required for your new air purifier.


Can you use a large air purifier in a small room?

Yes. There’s really no such thing as an air purifier that’s “too big” in terms of how well it purifies the air.

However, sound might be an issue. The larger the CADR rating, the bigger the fan and the more airflow. So, if you’re using a large air purifier in a small bedroom or office, the purifier might be noisy enough to disturb your sleep or work.

Are air purifiers worth it for Covid 19?

Possibly. Take all other precautions, and purchase an air purifier with:

A true HEPA air filter

1.5 to 2 times more capacity than “standard.” For example, the CADR Calculator at the top of the page says you need an air purifier with a minimum CADR of 500 for a space 750 square feet. In other words, if Covid is a concern, choose a purifier with a CADR or 750 (1.5 times higher) to 1000 (2 times higher)

What is the highest CADR air purifier

The highest CADR ratings you’ll find are between 550 and 600. CADR ratings of 588 are common in large air purifiers made from about 875 to 900 square feet.

What if I have more than 875 square feet of space?

Then you will need to purchase multiple air purifiers and set them up in zones of the home, apartment, office suite, retail space, warehouse, etc.

Is the smoke CADR different?

No, but again, we highly recommend choosing one with a HEPA air filter – not a HEPA-like or other filter type.

Two other options might be useful too. Look for an air purifier with an electrostatic function which helps to capture airborne smoke and other pollutants. Secondly, a charcoal pre-filter helps contain smoke and its odors.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree from Lone Star College and EPA & R-410A Certifications.

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