Air Changes Per Hour Calculator – Measure Your Air Exchange

Josh Mitchell

Written By

Josh Mitchell

Expert Reviewed By

Holly Curell

Last Updated On

“If you make a purchase using our provided links, we may receive a commission. Learn more here.

Key Takeaways

  • Air Changes Per Hour is a measurement of how many times the entire volume of air inside a certain room is replaced in an hour.
  • The higher the ACH, the more fresh air is being circulated every hour.
  • To calculate ACH, you need to know the dimensions of your room as well as the CFM of your air purifier or the exhaust fan.

I always ensure my home has proper ventilation and air circulation, and you should too. Having the right amount of external air entering your home helps keep your air clean and prevents a build-up of pollutants.

To ensure your home has proper ventilation, you must understand the air exchange rate and ACH (air changes per hour). What’s ACH? I thought you’d never ask!

Simply put, ACH measures how often an HVAC system can fill the room with air. Below I’ll dig deeper into ACH and give you an easy way to calculate it.

Air Changes Per Hour Calculator (CFM X Sq Ft)

My ACH calculator is the easiest way for you to find an accurate air exchange rate.

To use it, enter your room dimensions, room area (in square footage or length x width), ceiling height, and CADR (in cubic feet per minute) of the HVAC system in question, and you’ll get the ACH value!

Air Conditioner Lab ACH Calculator

How To Calculate ACH: The Air Changes Per Hour Formula

To calculate ACH, you need to know how many cubic feet your air purifier can clean per hour (CFM) and the area of space you intend to use it.

Calculate By CFM

Calculating air exchanges from cubic feet per minute is very easy, even more so with a digital calculator.

Before you jump ahead to calculate, make sure you have all the required measurements: CFM, room area, and height. Here’s a formula you can use to calculate ACH:

ACH = CFM x 60 / Area x height

CFM (cubic feet per minute) denotes air volume per minute, and to calculate air changes/hour, we have translated it into hours, hence multiplying by 60.

Here’s an example to get things cleared:

Suppose: CFM = 300, Area and height of room = 15’x20’x8′

Put that into the equation:

ACH = 300 x 60/ 15x20x8 = 7.50

Calculate By Room Size

Here’s a step-by-step process to calculate by room size:

  1. Use an ACH table to determine how many air changes are required for the room you intend to use your air purifier in
  2. Measure the volume of your space (including L’ x W’ x H’)
  3. Multiply the space volume with the required air changes
  4. Divide it by 60 minutes

Here’s an example:

Suppose: space volume = 18x24x10, ACH = 10. Put the values in the cubic feet per minute equation:

CFM = Space volume x ACH / 60 minutes

CFM = 18x24x10 x 10 / 60 = 720

How To Calculate CFM Of A Room

Determining the amount of air needed for a particular area distinguishes a healthy home and work environment from an unhealthy one.

Here’s how you can calculate the cubic feet per minute (CFM) of a room:

  1. Calculate the rooms’ length, width, and height
  2. Multiply all three measurements to find cubic footage of the space
  3. Multiply your room’s cubic volume by how many times you need indoor air to exchange or turn over in one hour
  4. Divide it by 60

What Exactly Is An Air Change?

Air changes per hour or better known as ACH or ACPH is the metric that measures how often an HVAC system can fill the room with air.

To put it simply, it’s a measure of the number of times the volume of air in the room is replaced with fresh air in an hour. Knowing the ACH is very useful when you have to compare air conditioners or air purifiers.

Example: Suppose you have a 250 CFM air purifier, and you have placed it in a 200 sq ft. space with an 8 ft. standard height ceiling.

So, how many ACH does the air purifier make?

Here’s how the cubic feet per minute measurement applies:

250 CFM x 60 minutes = 15,000 CFM

The volume of the space = 200 sq. ft. x 8 ft. = 1,600 cubic ft.

Air Changes per Hour (ACH) = (CFM x 60) / Volume of the space

Air Changes per Hour (ACH) = (250 CFM x 60 minutes) / 1,600 cubic ft.

Air Changes per Hour (ACH) = 9.4

Why Is Air Change Important In HVAC?

Homes and offices are designed to reduce power losses and optimize energy efficiency. As a result, they are often tightly enclosed to prevent conditioned air from escaping and reduce the energy required to heat or cool the space.

But did you know that workplaces and homes have now become prone to containments resulting from everyday dust to volatile organic compounds (VOCs)?

This happens due to inadequate air exchange! Insufficient air change, pollutants, irritants, and trapped allergens can lead to poor indoor air quality and endanger your loved ones’ health.

That’s why proper ventilation and air exchange are important strategies for improving indoor air quality. By bringing in fresh outdoor air and removing indoor air pollutants, a well-designed ventilation system can help to maintain healthy and safe indoor conditions.

ACH Of Different Air Purifier Brands

Alen Air Purifiers

Alen air purifiers are some of the best air purifiers currently available. They offer a large variety of air purifiers, but the BreatheSmart series is their best. They use an ACH from 2 to 5 times per hour, depending on the room size.

Coway Air Purifiers

Coway air purifiers are some of the smartest units available on the market. They barely make sound and automatically adjust fan speed according to the level of pollution in the air. Like Alen air purifiers, they also use 5 ACH and 2 ACH for different room sizes.

Molekule Air Purifiers

Molekule air purifiers use advanced technology to remove indoor air pollutants and can benefit allergy sufferers. However, although they provide recommended coverage areas, their lack of CADR and ACH values may make comparing their effectiveness to other air purifiers difficult.

Levoit Air Purifiers

Levoit air purifiers are exceptional options due to their compactness, three-stage filtering, and affordability. The best unit by Levoit uses 3.3 ACH in 463 sq ft. of room and a CADR rating of 360.

Okaysou Air Purifiers

Okaysou air purifiers provide you with clean indoor air quality. They offer an improved filtration mechanism that blocks all sorts of air pollutants. The most popular Okaysou unit uses 3 ACH with a coverage area of 500 sq ft. and a CADR rating of 210.

Dyson Air Purifiers

Dyson air purifiers are functional, stylish, and can be placed anywhere in a room. Though they are good at removing pollutants, they are the loudest option available. Additionally, they don’t come with a recommended coverage area, making it impossible to calculate their ACH value.

Winix Air Purifiers

Unlike most air purifier brands, Winix air purifiers offer a CADR rating of 200+ for all its units. This rating helps remove pollen, dust, and smoke particles from indoor air with relative ease; they use 5 ACH with a 246 CADR rating and a coverage area of about 360 sq ft.

Honeywell Air Purifiers

Honeywell is one of the most trusted brands on the market for air purifiers and other HVAC systems. Their best air purifier model uses 5 ACH in the 465 sq ft. of room and a 320 CADR rating.

Hathaspace Air Purifiers

If you’re looking for an air purifier, especially to remove smoke particles, you should choose Hathaspace air purifiers. They provide 2 air changes per hour in the 350 sq ft. of space.

GermGuardian Air Purifiers

GermGuardian air purifiers are known for durability and provide clean, quality air by removing all types of air pollutants. The best model by GermgGuardian provides 4 air changes per hour in 338 sq. ft. of room and a 218 CADR rating.

People Also Ask (FAQs)

What is considered a good ACH?

A good ACH is generally between 2 to 5. However, various factors, like the room area, help determine what a good ACH is.

How many air changes per hour should a home have?

A standard home in the United States should have between 5-6 air changes per hour. This helps ensure healthy air quality.

How long does it take to remove all air out a house?

It generally takes no more than 5 to 10 minutes to remove all the air from a house. You should air out your house regularly.

What is the minimum requirement for fresh air?

The minimum requirement for fresh air is 100 cubic feet per minute (CFM) for every 500 to 900 square footage of residential space.

How many CFM do I need for 1000 square feet?

A CFM between 240 to 260 is good for a 1000-square-foot room.


I understand the importance of clean indoor air and air purification. That’s why I monitor my air purifiers’ air changes per hour (ACH) rating to ensure adequate ventilation and air circulation.

Keeping a close eye on the ACH rating helps me ensure the air in my home is being refreshed frequently and effectively, reducing the presence of harmful pollutants and allergens.

Use my calculator at the top of this page to help you keep your family healthy and comfortable and give you peace of mind knowing you’re breathing safe and clean air.

Was This Article Helpful?

Josh Mitchell


Josh Mitchell
My name is Josh and I am obsessed with home appliances. From portable AC units to heaters and air purifiers, I enjoy testing, learning and using these devices to improve the air quality inside my family home.

My Favorite Home Appliance?

Midea U Shaped Window Air Conditioner

See Our Editorial Processes

Meet Our Team

Share Feedback