How to Calculate Air Changes Per Hour

How often do you ventilate your indoor space? You probably know already how important it is to regularly and properly air out your room (if not, read more about it in our previous article on how to air out your room). But do you know how well your space is being ventilated, and how much air is being exchanged?

Measuring air change may sound a bit strange since it seems unfeasible to measure something we can not even see or feel? However, there is a standardized procedure you can apply to find out whether your room is being properly ventilated, and whether you are getting enough fresh air in.

In this article, we will share with you our knowledge on how to calculate air changes per hour, explain why is it important to calculate it, provide you with some examples, and answer some of the most frequently asked questions on the topic.

What does air change Per Hour Even Mean?

Air change per hour also known as the ACH measure, air change rate, or air exchange rate, stands for a measure that tells us how many times has the whole indoor air volume changed for new air in a single hour. To put it in simpler words, this rate tells us how much clean air has entered our room in a single hour, and replaced the old air that was there already.

You may be thinking now that you are the best measure of how fresh your indoor air is, as you are the one who knows best whether it feels pleasant or not to be inside a particular room. While this can be considered true, it’s just a subjective feeling, but if you’d like to compare different air cleaning devices such as air purifiers or air conditioners, you will want some kind of standardized measure to help you which one is working better.

Therefore, ACH or air changes per hour metric, is super useful for comparing air purifying and air conditioning units, as it informs about the number of times has an HVAC unit cycled the volume of the entire room with clean air.

This is also extremely useful when it comes to air conditioners and air purifiers, as it can if there are not enough air changes per hour, you may need to consider a different unit.

Why Should We Calculate Air Changes Per Hour?

If you want to determine whether your space is receiving enough ventilation, air changes per hour can help you find that out. Also, if you want to use any kind of air purifier, measuring the air changes per hour will help you determine whether the device you’ve chosen is good enough for your particular living space, and if not, it can help you determine which type of device would be more suitable for you needs. Furthermore, it can also give you information on how many of these devices you actually may need.

For maintaining good indoor air quality, it’s extremely important to regularly ventilate your indoor space. If your indoor space is large, you may need more than one air purifying unit in order to achieve the best results.

Inside buildings, it’s very important to preserve good air quality. There are many negative consequences to improper ventilating of a building. For example, pollutants coming from both indoor and outdoor may start to build up indoors, and this can severely harm the health of people living in the space.

Bad indoor air quality can affect our health in several ways. For example, you can expect to experience irritation of the nose, eyes, and throat. This usually feels tingling and scratchy. Then, you may experience a mild to strong headache, feelings of dizziness, nausea, and fatigue. Finally, among the more severe consequences, there are heart disease, respiratory diseases, and also cancer.

Higher indoor pollution can also have psychological consequences, and this is something that we don’t talk about so often. For example, poor air quality has negative impacts on mood and productivity, which is can be a huge problem in workplaces and in schools, as both employees and students might experience lower levels of performance.

Since the COVID-19 pandemic started, the focus on fresh air and regular ventilation has become even stronger, as increased ventilation is one of the recommended ways of preventing the spread of the virus.

How to Calculate Air Changes Per Hour

In many cases, the distribution of indoor air is far from uniform and mixed. Depending on how efficient the airflow is and what is the ventilation method, the actual amount of air that is being exchanged will vary.

In a good case, you should have around 62% of air exchanged after one hour of ventilation. In order to equalize the pressure, the amount of air that is leaving your space and the amount of air that is entering the space should be approximately the same.

To calculate air changes per hour, you need to divide the volumetric flow rate of air in cubic feet per minute by the volume of the space expressed in cubic feet (length x width x height).

The formula looks like this:

ACPH = 60 Q / Vol

Q stands for the volumetric flow of air, while Vol stands for space volume.

Ventilation Rates

Usually, these are expressed as volume rates per person in an indoor space. We sometimes want to know this to be able to apply these measures more specifically in concrete situations when actual people are filling up the room.

The conversion rate between air changes per hour and ventilation rate per person can be calculated like this:

Rp = ACPH * D * h / 60

Rp is the ventilation rate per person, where we are calculating the cubic feet of air exchanged per person, while D stands for occupant density, or how many square feet each occupant is occupying. The letter h stands for the height of the ceiling.

Recommendations and FAQs about Air Change Rate

Air change rates are one of the most common metrics used when designing ventilation. Every type of space, e.g. laboratory, residential, or commercial, has its recommended standards for air change rates that will further guide the design of the ventilation.

For example, residential ones are calculated based on the size of the house or apartment and the number of people that live there.

For non-residential spaces, these rates are calculated based on the size of the floor area and the number of people who are/will be occupying the area, or alternatively, by calculating a dilution of the contaminants that are known to us.

Here are some examples of how much air changes per hour should be achieved per indoor unit.

For a residential basement, the air change rate should be between 3 and 4 units, while for a basement parking the number of units should reach between 15 to 30. In residential living rooms, the number of units should be between 6 and 7, while for the residential kitchens, it can go up to 7 to 8. For classrooms, it’s not recommended to go below 4 units, while in warehousing, depending on the size of the space, the recommended number of units can range anywhere from 3 to 10.

How to Increase Air Changes Per Hour?

To increase the number of air changes per hour, forced ventilation systems are generally used to circulate air, or an air exchange unit to refresh the indoor air, depending on the space.

Air Purifier Options

To get best results from your air purifier when it comes to this old air – new ratio rate, we strongly recommend you to always look for purifiers with HEPA filters. HEPA filters are designed in such a way to protect people working in a laboratories with potentially highly polluting and toxic chemicals. Protection proven is around 99%.

Today, most air purifying units designed for home use also have a HEPA filter. This filter will protect you from pollutants, and help keep the indoor air free of contaminants such as smoke.


Not many people are familiar with the air changes per hour rate, but knowing more about this metric can help you make a better choice for your home and make a smarter purchase when buying your air purifier or air conditioner.

Furthermore, this metric can help you determine whether or not you need to put more effort into ventilating your home and perhaps let some fresh air in through forced ventilation.

The metric itself is not difficult to calculate, all it takes are a few measurements of your indoor space – the height, the width, and the length of the walls, in order to calculate the volume.