top of page

Air Change Rate Calculator


Review the methodology to ensure it aligns with your project's requirements.

  1. Select English or SI Units

  2. Select Input Criteria - the calculator will solve for the other value. 

    • Air Flow Rate​

    • Air Change Rate

  3. Enter the room height

  4. Select how to enter the room area: 

    1. Enter the area directly​

    2. Enter the length and width of the room

  5. Click the Calculate button

  6. The results will be displayed in the results window

Air Change Rate
Air Change Rate Calculation Example

Methodology, Equations, and Example:

Air change rate, also known as air changes per hour (ACH), measures the

number of times the air within a space is completely replaced with fresh air in one hour. It is commonly used to assess and quantify the ventilation efficiency of a room or building.  The air change rate is calculated by dividing the total volume of air supplied or extracted from the space by the volume of the space itself.


The result is expressed as the number of air changes per hour. 

Q = ACH x 1hr/60 min x Room Volume


Q - Air Flow Rate [ CFM ( l/s )]

ACH - Air Changes per Hour  

Room Volume - L x W x H

Example: What volume of Air needs to be supplied to a 12'-0" x 12'-0" room that is 14'-0" high to achieve air changes per hour?

Q = (3  Air Changes/hr) x (1hr / 60 mins) x (12'-0" x 12'-0" x 14'-0") = 100.80 CFM (Cubic Feet per Minute)


The air change rate is important because it directly impacts indoor air quality and occupant comfort. Here are a few reasons why ACH is used:

  1. Ventilation and air quality: Higher ACH values indicate better ventilation and improved air quality. Increasing the rate of air changes helps remove indoor toxins, including odors, volatile organic compounds (VOCs), carbon dioxide, odors, and airborne contaminants. Fresh air is introduced while stale air is expelled, reducing the risk of respiratory issues and promoting a healthier environment.

  2. Temperature and humidity control: Proper ventilation through sufficient air changes helps maintain comfortable temperature and humidity levels. It prevents the buildup of excess heat or moisture, which can lead to discomfort, mold growth, and other issues.

  3. Controlling airborne pathogens: In settings such as hospitals, laboratories, and cleanrooms, controlling airborne pathogens is crucial. Higher air change rates help dilute and remove infectious particles, reducing the risk of spreading diseases.

  4. Compliance with building codes and standards: Many building codes and standards specify minimum air change rates for different types of spaces. These guidelines ensure that occupants have an acceptable air quality and comfort level. ACH values are used to verify compliance during building design, construction, and operation.

It's important to note that the optimal air change rate depends on various factors, including the size and function of the space, occupancy levels, and specific requirements or regulations. Engineering professionals and building designers consider these factors in determining the appropriate ACH for a particular environment.

bottom of page