Air purifiers come in all sizes. How to know what size air purifier do you need? There is a way to tell if you need a 100 CFM, 200 CFM, 300 CFM air purifier, and so on (based on CADR rating and ACH). To help everybody (because the calculations can be a bit complex), we have developed two useful and easy-to-use air purifier sizing resources, namely:

**Air Purifier Room Size Calculator**(basically a CADR calculator). You input the size of the space (square footage + ceiling height) and ACH (we will explain this further on), and the calculator will tell you what size air purifier you need (how many CFM CADR rating it should have).**Air Purifier Size Chart.**This is a complete chart for what size air conditioner you need for rooms between 100 sq ft and up to 2000 sq ft for 2 ACH and 5 ACH.*Example:*For a 300 sq ft room, you need an 80 CFM air purifier (at 2 ACH) or a 200 CFM air purifier (at 5 ACH).

Let’s first look at what determines the size of the air purifier you need. We only need to look for 2 factors:

**Size of your room or space.**We are not only looking for square footage (100 sq ft room, 500 sq ft room, 1000 sq ft space, and so on). We are looking for the amount of air that is inside that room. That means we have to include the*ceiling height*as well in order to calculate the total volume of air in the room in which we want to use an air purifier.**Air Changes per Hour (ACH).**At a minimum, an air purifier should be big enough to change all the air in the room at least 2 times per hour (that’sor air being changed every 30 minutes). For allergy-prone people, it is recommended to use*2 ACH*in the calculation of the size of air purifier needed (air changed every 12 minutes).*5 ACH*

Now, when we know how much air is in the room and want to change it **2, 3, 4, or 5 times per hour** (ACH is 2, 3, 4, or 5), we can calculate what size air purifier we need. As always, if you have any trouble finding out what size air purifier you need, you can use the comments below and we will help you out with the calculation.

*Example:* Let’s say we have a **500 sq ft room with an 8 ft ceiling height**. The total air volume of this room is 500 sq ft × 8 ft = 4000 cubic feet. If we want an air purifier to change all the air in this 500 sq ft room 2 times per hour, an air purifier will have to provide 8000 cubic feet of cleaned air.

*Where does the size of the air purifier (300 CFM, for example) come in?* Well, an air purifier with a CADR rating of 300 CFM will produce 300 cubic feet of air per minute. We have to convert that into cubic feet of air per hour (CFH). Since 1 hour has 60 minutes, we simply multiply it by 60. That means that a 300 CFM CADR-rated air purifier can produce 18000 cubic feet of air.

This air purifier will be too big for a 500 sq ft room because we only need 8000 cubic feet per hour (at 2 ACH). If we want to achieve 2 ACH in a 500 sq ft room, we only need an air purifier with a **133 CFM CADR rating**.

If, however, we want to get rid of allergies and thus want the air in the room to be changed every 12 minutes (5 ACH), we will need a bigger **333 CFM air purifier**.

Here is how the coverage area of air purifiers changes if we want only 2 ACH or more:

Here are easy **step-by-step instructions and formula** on how you can do this calculation of air purifier size manually (you can also use the Air Purifier Room Size Calculator and Air Purifier Size Chart further on but it’s useful to understand how the size of an air purifier for a room is calculated):

## How To Calculate What Size Air Purifier You Need?

The size of the air purifier is given by CADR rating. We would be talking about 100 CFM, 200 CFM, 300 CFM air purifiers, for example. Most manufacturers don’t include the airflow in the specification sheet, or even in the unit name.

*Example:* Coway Mega **400** is so named because it has a **400 CFM **CADR rating. Similarly, Honeywell HPA**300** gets its name by having a **300 CFM** CADR rating.

To calculate the size of air purifier we need, we will use this equation:

**Size Of Air Purifier (In CADR Rating)** = (Area × Height × ACH) / 60

Quick example: Let’s say we have a 1000 sq ft home with an 8 ft ceiling height and would an air purifier big enough to ensure 2 ACH. What size air purifier do we need for a 1000 sq ft home? Here’s how we can use the air purifier size formula to calculate that:

**Size Of Air Purifier (1000 Sq Ft)** = (1000 Sq Ft × 8 Ft × 2 ACH) / 60 = **267 CFM (CADR Rating)**

This is just one example. Here is the step-by-step guide on how to calculate the size of air purifier you need:

- Measure the length of the area where you want to use an air purifier.
- Measure the width of the area.
- Measure the ceiling height.
- Multiply these 3 numbers (length×width×height) in feet. This will give you the volume of the room; basically, the amount of air that needs to be purified.
- Decide on how many ACH or air changes per hour you need. The minimum recommended ACH is 2 and the highest one (for allergy-prone people) is 5.
- Multiply the amount of air in the room (Step 4) with the number of ACH (Step 5). You now have the size of the air purifier, but it is expressed in CFH (cubic feet per hour). You will need to convert CFH to CFM because the air purifier manufacturers specify the size of the air purifier – CADR rating – in CFM.
- To convert CFH to CFM, just divide the number from Step 6 by 60. You are now looking at what CADR rating should your air purifier have.

All these calculations are a bit complex. To simplify everything, you can use the air purifier sizing calculator below. Further on, we have also calculated the air purifier sizes for 100 sq ft, 150 sq ft, 200 sq ft, and so on up to 2000 sq ft spaces and summarized the results in a neat chart:

## Air Purifier Room Size Calculator

With this CADR calculator, it’s quite easy to determine the size of the air purifier you need for your space.

Let’s say you want to remove air particulates like dust, smoke, pollen, pet hair and dander, mold spores, and so on in a 300 sq ft room with 8 ft ceiling height. You are also an allergy-prone person and want to breathe the cleanest air (ACH = 5). You just slide the 1st slider to ‘300’, 2nd one to ‘8’ and the 3rd one to ‘5’ and you get the result:

For a 300 sq ft room with 5 ACH and 8 ft ceiling walls you need a **200 CFM CADR rated air purifier**.

Let’s look at the summarized air purifier sizing chart that lists air purifier sizes for spaces between 100 sq ft to 2000 sq ft and for ACH 2 or 5:

## Air Purifier Size Chart

In this chart, we assume that you have a standard ceiling height of 8 ft. Here are the sizes of air purifiers at 2 ACH and at 5 ACH (do read the note below the chart for big room air purifier sizes that need to produce 500 CFM or more CADR airflow):

Room Size: |
Air Purifier Size (CADR At 2 ACH): |
Air Purifier Size (CADR At 5 ACH): |

100 Sq Ft | 27 CFM | 67 CFM |

150 Sq Ft | 40 CFM | 100 CFM |

200 Sq Ft | 53 CFM | 133 CFM |

250 Sq Ft | 67 CFM | 167 CFM |

300 Sq Ft | 80 CFM | 200 CFM |

350 Sq Ft | 93 CFM | 233 CFM |

400 Sq Ft | 107 CFM | 267 CFM |

450 Sq Ft | 120 CFM | 300 CFM |

500 Sq Ft | 133 CFM | 333 CFM |

600 Sq Ft | 160 CFM | 400 CFM |

700 Sq Ft | 187 CFM | 467 CFM |

800 Sq Ft | 213 CFM | 533 CFM |

900 Sq Ft | 240 CFM | 600 CFM |

1000 Sq Ft | 267 CFM | 667 CFM |

1100 Sq Ft | 293 CFM | 733 CFM |

1200 Sq Ft | 320 CFM | 800 CFM |

1300 Sq Ft | 347 CFM | 867 CFM |

1400 Sq Ft | 373 CFM | 933 CFM |

1500 Sq Ft | 400 CFM | 1000 CFM |

1600 Sq Ft | 427 CFM | 1067 CFM |

1700 Sq Ft | 453 CFM | 1133 CFM |

1800 Sq Ft | 480 CFM | 1200 CFM |

1900 Sq Ft | 507 CFM | 1267 CFM |

2000 Sq Ft | 533 CFM | 1333 CFM |

As you will notice, for very large spaces at 5 ACH, you will need 500+ CFM air purifiers. You can check the biggest air purifiers for large rooms on the market here; most of them have at most or a bit over 500 CFM CADR rating.

Let’s say you have a 1200 sq ft space and want to breathe very clean air (ACH 5). That means you need an 800 CFM CADR rated air purifier. Now, an 800 CFM air purifier doesn’t exist. You will need to get a total of 800 CFM from two or more air purifiers. The most obvious combination here would be two 400 CFM air purifiers, for example.

Hopefully, now you understand how to calculate what size air purifier you need. On top of that, you can use the air purifier CADR calculator and this chart to adequately size your air purifier.

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