Air Purifier Room Size Calculator: What Size Air Purifier Do You Need?

Last Updated on April 30, 2024

If you are allergic and sensitive to the air around you, the best way to treat yourself is to get an air purifier to sanitize the air. Now, you might prefer getting the device right away and possibly thinking about what size air purifier do you need?

We cannot just tell you to get this or that. Generally, a small purifier covers a room that is 200 sq ft. Medium ones can cover up to 400 sq ft rooms, and the bigger ones can tackle 1500 sq ft. The CADR should hint at what room size it can cover. Using the air purifier size calculator, you can measure the CADR.

To get this rating, you will need to consider some important factors, such as the room size and the air changes per hr. (ACH). In this article, we will talk about how these variables are important and how you can determine the size of the purifier on your own. 

What Size Room Do You Have? How Much is Air in There?

The purifier should be powerful enough to cover every inch of your room; therefore, you better know the size of the living place. Moreover, you will need to figure out the amount of air in the room, not just the length and width of your space. 

To get the accurate air volume there, you must also consider the height of the ceiling. For example, suppose you have a room 10 feet by 12 feet. That means you have a 120 sq ft room. We just multiplied the numbers here. Now take the ceiling height; suppose it is 8 feet. 

The air volume of the room = 120 x 8 = 960 cubic feet. 

Know About the Air Cycling Capacity

Before getting your hands on a purifier, knowing about its ACH (Air changes per hour) is crucial. This reading identifies the purifier’s capability to change/filter the total air volume in a particular room within a span of 1 hour. 

For example, a 2 ACH purifier means it can filter the air every 30 minutes. This rating is what we call ideal. A 4 or 5-ACH purifier is recommended for those who have breathing conditions. The frequent air changes from a 5 ACH purifier will provide better breathability.  

What Is the CADR Rating on My Purifier?

Simply put, it means “Clean air delivery rate.” This is the rating that denotes how many particles the purifier can efface from the surroundings and the amount of air it can clean. So, the higher the reading, the better it can counter particulates in large areas.

Generally, the CADR rating will be labeled on the unit packaging. CADR ratings are certified by the Association Of Home Appliance Manufacturers. The values that you see are all generated under thorough tests conducted by AHAM.

For reference, do check out the figure below. 

aham certified air purifier
photo credit: ahamdir

Note: The reference image already highlights the maximum room size for this particular unit. Here, the CADR rating is noted as 80 for dust, pollen, and tobacco smoke. The reading in CFM tells you that this device can produce 80 cubic feet of air in 1 minute.

What size air purifier do I need for a bedroom? Finding CADR (With Example)

To achieve pure and clean air circulation, you will need a purifier that is fitting for your room. If you end up with a small purifier well below the cleaning capacity depending on the room size, you won’t get proper sanitization. To determine how big of an air purifier do i need, you can follow the steps below. 

Step 1: Know Where to Put the Purifier

First, you have to determine where you are going to place the purifier. Are you targeting a particular room or a bigger space like your whole house? 

If it’s a room, then the measurements can be extracted easily. But If it’s a house, then you might want to look at the housing plan. 

Step 2: Take Measurements of the Room

Now start by measuring the square footage of the available space. For example, you have a room that is 25 by 20 feet. Multiplying these numbers will give you the total sq ft of the room. The result will be 500 sq ft, which is the size of your room. 

Step 3: Determining the Volume of Air Within The Room

With just the length and the width, you get the room size. Now, considering the height as well will give you the exact value of the volume of air within that room.

We do this by measuring the height and then multiplying it by the size we got previously.

Suppose the height of your ceiling is 8 feet. Now multiply the variables (length, width, and height). 

Calculation: 25 x 20 x 8 = 4000 cu ft. 

Step 4: Pick the ACH

Purifiers are effective when they can cycle the air completely. Using the word cycle, we meant to say filter the contaminated air and produce cleaner output. ACH refers to air changes per hour. 

ACH can be rated as 2, 3, 4, or 5. Here, 2 means that the air changes twice every hour, at least once every half hour. This is the basic change rate for air purifiers. Those who are allergic to dust and pollutants might want to bump the numbers up and prefer 5 ACH. Here, filtered air is circulated five times in one hour, giving you fresher air to breathe. 

Step 5: Size of Purifier – CADR Rating

This is the most important step that will ultimately show you what size of purifier you need for your room. You will need to find the CADR in this step.

The formula for CADR evaluation: (Volume of air) x (number of ACH) 

As per our example, the outcome should be: (4000 cu ft.) x (2) = 8000 CFH

Note: CADR is measured in cubic feet per minute and not per hour. So, to convert it, you will have to divide it by 60. So this is how it goes: 8000/60 = 133.3 CFM

Takeaways: For a room that is 500 sq ft, 8 feet in height, you will need an air purifier that is 133.3 CFM. This should filter the air twice within an hour. If you want to circulate the air better, then 4 or 5 ACH should be your priority. In that case, for 5 ACH, you will need a 333.3 CFM purifier.  

Calculating Airchanges Per Hour

Suppose you have a purifier with a 300 CFM rating. Can you determine how many times the air changes in one hour? Yes, you can!

Here is a possible scenario: Imagine you have a 250 sq ft room with 9 feet of height. That means your room volume is 2250 cu ft. 

Here is what you need to do: 

ACH = The air change per hour (cu ft.) / volume of the room in (cu ft.)

The calculation will be as such: (300 x 60) ÷ (2250) = 8 ACH

A rating of 8 ACH means your purifier will cycle the air 8 times in one hour. 

Note: The recommended value of ACH for allergy and asthma patients should be a minimum of 4. A rating of 2-3 can work for those who are in a home environment and don’t have any breathing conditions. 

video credit: VRF Wizard

What Size Purifier Will Work for a 1000 Sq Ft Area (8 Ft height)?

This example is just to make you understand better. Imagine you have a 1000 sq ft studio room with an height of 8 feet. You can easily find the much-needed CADR for your new purifier. 

Size Of Purifier: (Volume of Air) x (air change per hr.)

1000 x 8 x 2 = 16000 CFH

16000/60 = 266.6 CFM

You will need a purifier with a 266.6 CFM minimum to sanitize the air properly in a 1000 sq ft room.

Frequently Asked Questions

  1. How do I know what size air purifier do I need?

When you purchase air purifiers, you can check the ratings on the packaging label. The AHAM-verified CADR ratings and the minimum room size are mentioned for you.

If that is not on the cards, you can calculate the CADR by multiplying the room volume with the ACH. Make sure you get the output in CFM. Read the article above if you are still confused about calculating CADR or ACH. 

  1. Is it better to have one large air purifier or multiple small ones?

We suggest you get a purifier, depending on your room size. One can work wonders if everything lines up. For rooms with complex designs, you might need 2 purifiers for better cleansing. Placing 2 purifiers in different locations in your room can improve ventilation. Knowing where to place an air purifier can be helpful for you. 

  1. Does room size matter for air purifiers?

Yes, size does matter, affecting how the purifier works. The quality of purification will vary depending on room size. For bigger rooms, you can’t just use small purifiers. For example, if you get a purifier with a 105 CFM rating, 2 ACH, and use it in a 1000 sq ft room, then the purification won’t work properly. 

You will need a 266.6 CFM purifier minimum for 2 ACH. The CFM reading must be higher if you want more air cycles per hour.  

  1. How much square footage does an air purifier cover?

Air purifiers work in different size rooms. It can be a small space or a large room. And you will likely find a purifier for every setting. HEPA small version; it can purify up to 250 sq ft. For bigger rooms, you might want to look at the Modify MA 112; it is well suited for a 2500 sq ft room. 


Getting an air purifier for your house will promote healthy living, there is no doubt, but if you mismanage the size of the purifier, then the efficacy might not be optimal. What size air purifier do you need to sanitize the air properly? 

You have to look at the CADR reading on the purifier and the corresponding room size. The values should be mentioned on the packaging. Just abide by it, and the purifier should work fine. 

Keep in mind that you can estimate the purifier size by yourself. The room measurement is critical here. You must calculate the air volume within the space and then integrate it with the ACH to get the desired CADR. 

Photo of author

Paul M Walker

I, Paul M Walker, am the founder and the author of this little site you are currently on. I work to provide readers with no-fuss and easy-to-follow solutions regarding common air purifier and humidifier problems.

2 thoughts on “Air Purifier Room Size Calculator: What Size Air Purifier Do You Need?”

  1. Great video! Very clear. Now I can start my search. Do you have any suggestions as to the best and most reliable units?

    Thank you


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