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?
- What is a Good CADR Rating for an Air Purifier?
- Tools to Determine the Right CADR Rating for an Air Purifier
- Clean Air Delivery Rate - CADR Table
- Air Changes per Hour with Air Purifier by Room Size
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?
Plug in your room size, and our Pick HVAC CADR Calculator shows the Minimum Required CADR if you want one air change per hour.
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 ft||134 cfm|
|300 sq ft||200 cfm|
|400 sq ft||267 cfm|
|500 sq ft||334 cfm|
|600 sq ft||400 cfm|
|700 sq ft||467 cfm|
|800 sq ft||534 cfm|
|1,000 sq ft||667 cfm|
|1,200 sq ft||800 cfm|
|1,500 sq ft||1,000 cfm|
|1,800 sq ft||1,200 cfm|
|2,000 sq ft||1,334 cfm|
|3,000 sq ft||2,000 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:
|AIR CHANGES PER HOUR (ACH)|
|CADR||1 ACH||2 ACH||3 ACH||4 ACH|
|50 CFM||78 sq ft||39 sq ft||26 sq ft||20 sq ft|
|100 CFM||155 sq ft||78 sq ft||52 sq ft||39 sq ft|
|125 CFM||194 sq ft||97 sq ft||65 sq ft||49 sq ft|
|150 CFM||233 sq ft||116 sq ft||78 sq ft||59 sq ft|
|200 CFM||310 sq ft||155 sq ft||104 sq ft||78 sq ft|
|240 CFM||372 sq ft||186 sq ft||124 sq ft||93 sq ft|
|300 CFM||465 sq ft||233 sq ft||155 sq ft||117 sq ft|
|350 CFM||543 sq ft||271 sq ft||181 sq ft||136 sq ft|
|400 CFM||620 sq ft||310 sq ft||207 sq ft||155 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.