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Air Changes per Hour (ACH) Calculator

Air changes per hour, denoted as ACH, is a metric that measures how many times an air purifier or HVAC system can replace the air in a room. You will need to use this metric to determine whether or not an air purifier can meet your air-turnover needs.

In today’s guide, we’ll briefly go over how you can calculate the ACH of an HVAC device on your own.

Air Changes per Hour Calculator

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ACH Calculator and Formula

Here is a simple calculator you can use to calculate how many ACH your air purifier or HVAC system can provide.

Alternatively, you can attempt to calculate ACH manually. Here’s how you do it.
ACH = (CADR × 60) ÷ (Room Size × Room Height)


  • ACH = Air Changes per Hour
  • CADR = Clean Air Delivery Rate, aka Cubic Feet per Minute (CFM)
  • Room Size = Room Length × Room Width
  • Room Height = The height between the room’s floor and ceiling

In the first half of the equation, we convert CADR, or the air purifier’s cubic feet per minute output, into hours by multiplying it by 60 minutes. The second half of the formula calculates the volume of a room where your will plant your air purifier/HVAC device.

Dividing the hourly cleaning output of the air purifier/HVAC device over the total volume of an enclosed room gives us a final ACH count, which we can use to determine whether or not the device in question is suitable for the room.

It’s also worth noting how many ACH you will need based on room types. The following chart will assist you in finding the right air-purifying device for a specific room at home or business.


  • Basements – 3-4 ACH
  • Bedrooms – 5-6 ACH
  • Bathrooms – 6-7 ACH
  • Living Rooms – 6-8 ACH
  • Kitchens – 7-8 ACH
  • Laundry Room – 8-9 ACH


  • Offices – 6-8 ACH
  • Cafeteria/Break Rooms – 7-8 ACH
  • Meeting Rooms – 8-12 ACH
  • Smoking Area – 13-15 ACH


  • Dining Area (Non-Smoking) – 8-10 ACH
  • Dining Area (Smoking) – 15-18 ACH
  • Kitchens – 14-18 ACH

ACH formula

Did you Know

In restaurants that allow smoking, cigarette smoke may be present in all areas of the restaurant, even if it the restaurant has a designated smoking section

Let’s say you are in search of an air purifier for your bedroom. The bedroom measures 10 × 10 in size and has 8-foot-tall ceilings. The air purifier you’re looking at has a CADR of 150 in all three categories (dust, smoke, pollen).
If we input these variables into the ACH formula, we should get:

  • ACH = (CADR × 60) ÷ (Room Size × Room Height)
  • ACH = (150 × 60) ÷ (100 × 8)
  • ACH = 9,000 ÷ 800
  • ACH = 11.25

The air purifier you’re considering has a maximum ACH output of 11.25 in your 10 × 10 × 8-foot bedroom, which is more than twice the necessary minimum ACH requirement for a bedroom.
Alternatively, if you want to calculate the minimum CADR to determine whether or not it will fit your desired ACH level, you can use the following formula:

CADR = (ACH × (Room Size × Room Height)) ÷ 60

Assuming you want to purchase an air purifier that can produce at least 5 ACH in a 10 × 10-foot room with an 8-foot-tall ceiling, the calculation would be as follows:

  • CADR = (ACH × (Room Size × Room Height)) ÷ 60
  • CADR = (5 × (100 × 8)) ÷ 60
  • CADR = 4,000 ÷ 60
  • CADR = 66.667

So, an air purifier/HVAC device that produces a minimum CADR of 66.667 CFM would be suitable enough to clean the air in your 10 × 10 × 8-foot bedroom.

Levoit Core 400S
Levoit Core 400S

Choosing the Right Air Purifier

Now that we know what ACH is and how to calculate it. But should you just calculate how many times an air purifier can clean the air in a room, or is there more to it?

How to Find the ACH of an Air Purifier?

  1. Calculate The Room Size: Length x Width x Height = Room Size
  2. Identify The Purifier’s CADR: The CADR, or Clean Air Delivery Rate, is provided by the manufacturer. Look for the tobacco smoke CADR.
  3. Convert The Cadr To Cubic Feet Per Hour: If the CADR is provided in cubic feet per minute (cfm), multiply the number by 60 for cubic feet per hour (cfh).
  4. Calculate The Air Changes Per Hour: Divide the CADR amount (in cfh) by the room size to find the air changes per hour (ACH). The ideal ACH is 5 or higher.

There’s more to an air purifier.

Coway Airmega 400
Coway Air Purifier

When shopping for an air purifier or a type of HVAC device, you should keep your eyes peeled for the following:

  • CADR: This metric measures the device’s cleaning efficiency. The higher the CADR rating, the more air changes it can pack per hour.
  • Filtration States: How many filters does the air purifier come with? Standard air purifiers usually have two or three stages, but some of the top-tier models have four stages and beyond.
  • Coverage Capacity: Just because an air purifier can theoretically produce 5 ACH in a room doesn’t mean it’s equipped to handle it. So, pay close attention to an air purifier as its coverage capacity (expressed in square feet) will give you an idea of what type of room it belongs in.
  • Air-Quality Sensors: Some air purifiers come with highly sensitive sensors that can calculate how polluted the air in a room is. If pollution concentrations exceed a certain threshold, the air purifier will kick it into high gear without human intervention.

Air Recommended Air Purifiers

Our Pick Perfect for bedrooms, playrooms, and living rooms, the Coway Airmega AP-1512HH is one of the best-performing, most durable, and most economical purifiers we’ve tested.

Most Popular The Levoit Core 300 air purifier has some strong suits, but in our opinion it falls short of carving out a meaningful niche for itself. Its ideal use would be in a small bedroom occupied by someone who is sensitive to noise.

Also Great With high end air cleaning performance and a whisper quiet fan, the Winix 5500-2 is the perfect combination of power and finesse. It cleared airborne particulates out of our smokey testing room faster than any other model.

Best of the Best If you have a large space you want to remove irritants from, We recommend the Airmega 400S. All of Coway air purifiers do an excellent job of attacking indoor pollutants, and we highly recommend any of them. The 400S is designed to accommodate large room sizes.

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About the author

Hi I’m Alex, founder of HouseholdMe.com and I’d like to say thank you for dropping by. Like most of you, the first thing I look at before buying something online is reviews or buying guides. By reading what other people say will help me gauge whether or not a product is good or not.  I am trying to help people find answers, solve problems, and get inspired.

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