Air Changes per Hour (ACH) calculator

This web page allows you to estimate the Air Changes per Hour (ACH) of a given space subject to natural or mechanical ventilation. This ACH Calculator is based on, and is a companion of, the research paper:

"NEW METHODOLOGY TO EVALUATE AND OPTIMIZE INDOOR VENTILATION BASED ON RAPID RESPONSE SENSORS."
M. Durán del Amor, A. Baeza, F. Esquembre, and M. Llorens Pascual del Riquelme (2024 currently under review).

Internationally recognized institutions have issued guidelines with intervals of recommended numbers of air changes per hour (ACH), for different types of installations and uses, that will typically guarantee good indoor air quality as well as protection against viruses, in general. These recommended intervals are aimed at the design of ventilation systems (which explains why the intervals are, in many cases, rather wide) and national bodies use these guidelines when issuing regulations for new constructions. However, having installed a correctly dimensioned ventilation system does not guarantee an effective nor efficient everyday use. The recommendations alone, bluntly taken, do not consider factors such as the air volume of the space or its actual occupancy.

A second important question is that of compiling the various, some new, recommendations issued by different organisms concerning the maximum concentrations of different pollutants or the recommended respiration flow rate per person (the amount of air that should be made available to a person) to ensure good air quality and protection against some particularly dangerous viruses, such as SARS-CoV-2 coronavirus.

Since renewing air has both health and economic implications, it is advisable to have an easy to apply methodology to assess the actual ventilation taking place in each space at a given time, considering the characteristics of the space, its occupancy, the activity inside it, the ventilation strategy, and the outside concentration of pollutants (from which the renovation air is being taken). This is the main goal of this freely accessible Web page.

This web tool provides a methodology to easily evaluate the efficiency of the existing ventilation in different scenarios to fulfil the recommended minimum air changes per hour to ensure good ventilation and avoid contagion of COVID-19. It includes a quantitative method to estimate the real air changes per hour of a public space from simple CO2 experimental measurements, the volume of the space, its occupation, and the activity inside it.

Therefore, this web app can help managers of public concurrence spaces achieve the required balance between safe and healthy indoor air, and occupant’s comfort in the most energy efficient way.

For more details, consult the above mentioned research paper.

In order to perform the ACH calculations, you'll need to fill in the information requested in the Calculator tab of this Web page. The information must be collected by you and requires some experimental data, such as the CO2 concentration inside and outside the space.

If you are one of those who like to plunge into the computation, just go to the Calculator tab and enter data. The process should be easy to follow. If you prefer to read some instructions first, visit the User's guide tab now.

How to use the calculator

Short answer: Change to the Calculator tab and fill in the experimental data requested. When done, click the Click for results banner at the bottom to show/hide the computation results.

Please notice that many labels show an icon to their left. Click on this icon for information about this field.

Note: The input area for some fields display a red asterisk at the end of their labels, and show the input field with an orange background when the data hasn't been entered. These correspond to information required to do the ACH computation. Other fields are optional, but will allow the calculator to offer additional information, such as the ACHac, the ACHmv, and maximum occupancies under different conditions.

Longer answer: In the Calculator tab, proceed as follows:

  1. Fill all the data requested about the space. As a minimum, the volume is required.
  2. A table allows you to specify the type of occupancy of the space and the activity of the people in it. Notice that this information is not used to compute the number of people in the room, but to categorize them by age ranges, gender, and activity. The figures you enter are treated as relative to the sum of all the cells. This means that you can, as well, enter percentages. Even non-integer numbers.
    This information is then used to estimate the CO2 Average Exhalation Rate, which is required to compute the real ACH. (Do not try to write in this field. Fill at least one of the cells in the table, instead.) Notice that the table alows you to detail the occupancy by age ranges and gender... if you do not have this information, just use the Average person row.
  3. Finally, fill the number of people present in the room when you took the measurements (Occupancy), and the concentration of CO2 you measured inside and outside the space.

And that's it!

Click the Click for results banner at the bottom to show/hide the computation results.

Again, please notice that many labels show an icon to their left. Click on this icon for information about this field.

Information about the space
The volume space is a required piece of information. Measure the studied space dimensions - width, length, and height - in metres and multiply them to calculate the air volume. If measuring with a laser, place the meter as close as possible to the walls and floor and take special care to ensure that there are no objects in the way that could diffract/deflect the laser.
Although the type of space is not required to compute the real ACH for the space, this information is useful to compute ACHac, the recommended ACH to help prevent COVID contagion for given occupancies, and ACHmv, the recomended ACH for a minimum ventilation.
The maximum occupancy is only required to compute the maximum ACHobj required at that occupancy.
The table below is used to provide information about the typical activity of the people inside the space. The numbers in the table cells are used to find the average exhalation rate per person. This magnitude (required for our computations), together with the actual or expected occupancy, gives the estimated CO2 generation rate.
The numbers are relative. This means that 9 people at rest and 1 walking gives the same exhalation rate as 90 - 10.
Activity Rest Walking Physical activity
Age \ Gender Male M FemaleF Male M FemaleF Male M FemaleF
11-15 years
16-20 years
21-29 years
30-39 years
40-49 years
From 11 to 49 years
Average person
Average Exhal Rate (l s-1 ) *
Experimental measurements
The total number of people inside the space during the measurements. This number should be constant during the experiment.
The CO2 concentration measured inside the space.
After swiching on the measuring device, wait for at least 10 minutes to ensure that the levels of pollutants monitored by the meters stabilize. After that, record the concentration values for another 10 minutes. With the data obtained, compute the mean. Enter this mean here.
The CO2 concentration measured outside the space.
After swiching on the measuring device, wait for at least 10 minutes to ensure that the levels of pollutants monitored by the meters stabilize. After that, record the concentration values for another 10 minutes. With the data obtained, compute the mean. Enter this mean here.