Air Changes Per Hour Calculator: How to Calculate Air Changes Per Hour

Breathe easy in every room of your house

Woman checking notes and using her phone to do some calculations
Photo: staticnak1983 / E+ / Getty Images
Woman checking notes and using her phone to do some calculations
Photo: staticnak1983 / E+ / Getty Images
Deane Biermeier
Written by Deane Biermeier
Contributing Writer
Updated Jun 24, 2025
Highlights

  • The number of air changes needed per hour varies by room and uses.

  • The ideal air change per hour range is between three and five.

  • Know the volume of space and cubic feet per minute value from your HVAC system. 

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Learning how to calculate air changes per hour (ACH) can help you get a grasp on your HVAC performance and help you improve your indoor air quality, if necessary. You'll need to know the cubic square feet of the area and the cubic feet per minute (CFM) of your HVAC system. Use this air changes per hour calculator to determine your current ACH and help you plan for changes.

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How Many Air Changes Per Hour Are Needed?

The ideal ACH rate for typical homes is between three and five. That means the entire volume of air within the home is replaced three to five times each hour. However, rooms such as bathrooms, kitchens, and basements often require higher ACH rates to help reduce humidity and pollutants in the indoor air.

In addition to the regular ACH rates your HVAC is capable of, you can pay for the cost of a blower door test to determine the amount of air leakage your home has. The HVAC pro can include this value in their calculations when determining your home's overall ACH rate.

ACH rates can also apply to single rooms, stories, or individual portions of your home. The chart below shows the average square feet that various HVAC airflows can handle, assuming a healthy ACH of between three and five and a standard ceiling height. 

HVAC Airflow in Cubic Feet Per MinuteArea Volume in Square Feet
40–70100
80–140200
200–340500
400–6701,000
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How to Calculate Air Changes Per Hour

To calculate ACH, you'll only need to know the volume of the space in question and the current or proposed CFM of the HVAC system. Alternatively, you can use the calculations the other way to determine if you need to increase your vent airflow or repair or replace your air conditioning system. 

To determine ACH, measure the room or space area and multiply it by its height to determine its volume. Then, multiply the CFM of the HVAC system by 60 to represent the number of minutes in an hour. Divide that value by the volume of space. The result is your ACH rate. 

Air Changes Per Hour = Cubic Feet Per Minute x 60 / Room Volume

How to Measure Space Volume

Bright open plan living room and dining area
Photo: piovesempre / iStock / Getty Images Plus

To measure the volume of a space, room, or home, determine the square foot or area measurement and multiply it by its height. If you're calculating for multiple rooms, determine the volume measurement for each room and add them together for your total. 

How to Measure Volume of Irregular Spaces

To measure the volume of irregularly shaped areas, divide the space into manageable chunks with rectangular, triangular, or semi-circle shapes to calculate the square footage. If the space has a sloped ceiling, use the average ceiling height as your value. Determine the average ceiling height by adding the lowest and highest values and dividing them by two.

3 steps to measuring the volume of an irregular shaped room illustrated, including dividing the room into smaller more regular shapes

Who to Hire to Calculate Air Changes Per Hour

To help maintain an adequate ACH level in your home, changing your furnace filter regularly is essential. Not only does doing so allow for smooth airflow, but it can also help prolong the lifespan of your HVAC system. 

If you have concerns about the performance of your system, it's a good time to call an HVAC repair professional. When hiring your HVAC technician, keep in mind that local pros will know the best ACH levels specific to your location and your home.

Frequently Asked Questions

Healthy air change per hour, or ACH, rates vary by room, usage, average occupancy, and volume. The minimum ACH that's considered healthy averaged throughout the home is 0.35. However, in rooms where humidity is an issue, such as bathrooms and kitchens, healthy ACH levels are between eight and 15.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers, or ASHRAE, sets the standard guidelines for air changes per hour in homes and businesses. The International Residential Code, or IRC, also sets ACH requirements for residences and bases those requirements off of ASHRAE standards plus climate specifics related to your location.

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Deane Biermeier
Written by Deane Biermeier
Contributing Writer
Deane has spent 30+ years in residential construction, contracting, remodeling, maintenance, and home repair, and now contributes DIY and educational articles for publications like Forbes Home and U.S. News. Throughout his career, he has strived to help homeowners find the potential hidden within the walls of every home and empower them to take control of their household repair and remodeling endeavors.

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