Air Changes per Hour Calculator

Created by Julia Kopczyńska, PhD candidate
Reviewed by Anna Szczepanek, PhD and Rijk de Wet
Last updated: Jun 12, 2023

Knowing how to calculate air changes per hour can be helpful, especially if you want to know how often you receive a new load of fresh air from your air device. With this air change per hour calculator, you can easily determine how often the whole room air is exchanged: all you need to know beforehand is the area and height of your room and the CFM of your air device.

Scroll down to find an explanation of what ACH is all about and what values you should aim for. We'll also explain the air changes per hour formula.

What does ACH mean?

ACH stands for air changes per hour and provides information on the hourly ratio of air exchanged within a room. Generally, ACH should vary between 4–30 depending on the type of room and its purpose. Higher ACH should be granted for:

  • closed spaces — garages, manufacturing halls
  • spaces with increased humidity — kitchens, bathrooms
  • gathering places — malls, conference rooms, classrooms
  • rooms storing temperature- and humidity-sensitive materials and products — storage houses, pharmacies
  • places of increased infection risks — hospitals, laboratories

Thus, the question of how to calculate air changes per hour is a practical matter in various settings.

What does ACH tell us?

Knowing the ACH for a given space can be relevant in many ways. Proper ventilation substantially influences our well-being and health, as air pollutants and microbes do not remain within one area, since that air is being exchanged. Some spaces, such as kitchens and bathrooms, require more ventilation and thus a higher air exchange ratio, but also public areas, such as hospitals and schools, need to be adjusted to the higher exposure to people and germs. Higher usage of O2 and the consequential higher emission of breathing CO2 is an important factor in this context, which you can also access with our CO₂ breathing emission calculator. A proper ACH ratio is, therefore, crucial to ensure good air quality and properly regulate humidity.

💡 You may also be interested in finding the suitable air conditioner size for your room with our air conditioner room size calculator.

How to use this ACH calculator

Before we come to the formula for calculating air changes per hour, let's get familiar with the variables needed for this kind of calculation:

  1. Since you want to calculate air changes per hour for a particular room, you will need to know its volume.

  2. The next step is to check for the CFM of your air device. CFM stands for cubic feet per minute and is the standard metric for airflow.

  3. That's it! Our tool immediately computes ACH based on your input.

💡 If you are interested in calculating CFM, check out our CFM calculator!

How to calculate air changes per hour?

To compute ACH by hand, follow these steps:

1. Determine the volume of the room

  • If you know its area, you need to multiply it by the height of the ceiling.
  • If you do not know the area, measure the length and width of your room and multiply them together to get the area.

💡 You can also use our area of a rectangle calculator and volume calculator for this step.

2. Adjust CFM

  • To calculate the value for the air exchange per hour, you will need to multiply the CFM of your device by 60 (because 60 minutes = 1 hour).

3. Calculate ACH

  • Finally, divide the adjusted CFM value by the room volume.

In a nutshell, we have the following formula for air changes per hour:

ACH=CFM×60Area×Height\small \mathrm{ACH} = \frac{\mathrm{CFM} \times 60}{\mathrm{Area} \times \mathrm{Height}}
Julia Kopczyńska, PhD candidate
Air changes per hour
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