Informational Guide

Air Changes Per Hour Explained

It’s essential to understand ACH to prevent a build-up of pollutants inside your house. But what exactly is air changes per hour? and how do you calculate it?

by Josh M

Every home in America should have proper ventilation and air circulation. Having the right amount of external air entering your home or workplace prevents a build-up of pollutants.

It also helps control humidity levels, temperature, odor, and various other elements that can affect the comfort and health of those living inside.

Therefore, it’s essential to understand ACH to prevent this build-up of pollutants inside your house. But what exactly is ACH, or air changes per hour?

Read on as we elaborate:

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 air volume removed or added to a room in an hour divided by the room’s volume. 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 CFM measurement applies:

250 CFM x 60 minutes = 15,000 CFM

The volume of the space is 200 sq ft. and 8 ft, therefore:

200 sq ft x 8 ft = 1600 ft

Put the above two in the ACH equation:

ACH = Volumetric air / total volume of space

ACH = 15,000/1600 = 9.3 times


Why Is Air Change Important In HVAC?

Homes and offices are made to reduce power losses and optimize efficiency. In addition to that, they are tightly enclosed to keep the conditioned air inside for an extended period.

But did you know that workplaces and homes have now become more 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 degrade indoor air quality and endanger your loved ones’ health.

However, with proper air change and a ventilated system, you can minimize these pollutants and live and work in safe and healthy conditions.

How To Calculate ACH: The Air Changes Per Hour Formula

To calculate ACH, you need the CFM of your air purifier and the area of space you’ll intend to use it.

Calculate By CFM

Calculating ACH from cubic feet per minute is very easy, even more so with a digital calculator. But 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 denotes air volume per minute, and to calculate air changes/hour, we have translated it into hours, hence multiplying with 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 required air changes
  4. .Divide it by 60 minutes

Here’s an example:

Suppose: space volume = 18x24x10, ACH = 10. Put the values in CFM equation:

CFM = Space volume x ACH / 60 minutes

CFM = 18x24x10 x 10 / 60 = 720

Air Changes Per Hour Calculator

It is also possible to use ACH calculators to find an accurate air change rate. 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!

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 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 rooms’ cubic volume by how many times you need indoor air to exchange or turn over in one hour
  4. .Divide it by 60
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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 are ideal for allergy sufferers. These units remove air pollutants from indoors and destroy them. Although they provide recommended coverage area such 600 sq ft. and 1000 sq ft, however, offers no CADR or ACH value.

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 (FAQ)

What is considered a good ACH?

Typically, anything between 2 to 5 ACH is considered a good air change; however, this also depends on the room area.

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 at most for healthy air quality.

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

You should air out your house regularly as it takes no more than 5 to 10 minutes to remove all air.

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 the residential space.

How many CFM do I need for 1000 square feet?

Anything between 240 to 260 CFM is good for 1000 square feet of room.


Air changes per hour is an essential system of measurement through which you can know how often your air purifier can fill the room in a specified time.

It ensures better and safe air quality and helps your family stay healthy.

Last Updated on July 16, 2021

Josh M

My name is Josh and I am obsessed with the HVAC industry. I created this website to help HVAC techs of all levels get the best out of their heating & cooling systems. I have spent thousands of hours studying air conditioners, heaters and home air products so you can learn & buy with confidence. Learn more about the team here.