Air Change Rate Calculator

Units
You know the
Floor given as

Results

Enter the air changes per hour.

Q = ACH × room volume ÷ 60 in US units (cfm, ft³), or Q = ACH × room volume × 1000 ÷ 3600 in SI (l/s, m³).

How to use it

Review the methodology below to make sure it aligns with your project's requirements, then:

  1. Choose US (ft, cfm) or SI (m, l/s) units. Switching converts the values already entered.
  2. Choose what you know: the air changes per hour, to find the air flow rate, or the air flow rate, to find the air changes per hour.
  3. Enter that value and the room height.
  4. Give the floor as length × width, or enter the floor area directly.
  5. The room volume, air flow rate and air changes per hour appear in the results and update as you type.

Use Copy link to this result to save or share the calculation.

Methodology, equation 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 in one hour. It is commonly used to assess and quantify the ventilation of a room or building. The air change rate is calculated by dividing the volume of air supplied to or extracted from the space in an hour by the volume of the space itself, and is expressed as the number of air changes per hour.

Q = ACH × 1 hr / 60 min × Room Volume

Where:

  • Q: air flow rate, CFM (l/s)
  • ACH: air changes per hour
  • Room Volume: L × W × H, ft³ (m³)

In SI units the same relationship is Q [l/s] = ACH × Room Volume [m³] × 1000 / 3600.

Example: What airflow needs to be supplied to a 12'-0" × 12'-0" room that is 14'-0" high to achieve 3 air changes per hour?

Q = (3 air changes/hr) × (1 hr / 60 min) × (12'-0" × 12'-0" × 14'-0") = 100.80 CFM (cubic feet per minute)

Why the air change rate matters

The air change rate directly affects indoor air quality and occupant comfort. ACH is used for:

  • Ventilation and air quality. Higher ACH values indicate more ventilation. Increasing the rate of air changes helps remove indoor pollutants, including odors, volatile organic compounds (VOCs), carbon dioxide and airborne contaminants. Fresh air is introduced while stale air is expelled, reducing the risk of respiratory issues and promoting a healthier environment.
  • 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.
  • 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 disease.
  • Compliance with building codes and standards. Many building codes and standards specify minimum air change rates for different types of spaces, so that occupants have acceptable air quality and comfort. ACH values are used to verify compliance during building design, construction and operation.

The right air change rate depends on many factors, including the size and function of the space, occupancy, and the specific requirements or regulations that apply. Engineers and building designers weigh these factors to determine the appropriate ACH for a particular environment.

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