**ACH** or * Air Changes Per Hour* is a metric that tells us how many times an HVAC device can fill up the full volume of a room with air. This is especially useful when comparing different air purifiers or air conditioners.

*Example:* Let’s take an air purifier with 250 CFM airflow. We put it in a 200 sq ft room with the standard-height ceiling (8 ft). How many air changes per hour does the unit make?

*Calculation:* 250 CFM is 250 cubic feet per minute. In one hour (60 minutes), we get 60*250 = 15.000 cubic feet per hour. The whole volume of the room is 200 sq ft * 8 ft = 1.600 cubic feet. Such an air purifier is capable of changes the whole volumetric air in room 15.000/1.600 = 9.375 times.

*Answer:* **ACH = 9.375**

Here is a neat air change per hour calculator you can freely use. Just put in the area, ceiling height and CFM of an HVAC device in question and you can calculate ACH:

## Calculator

## Formula (Calculate ACH Yourself)

The formula of how to calculate air changes per hour from CFM is simple enough. Pretty everybody can calculate it using a digital calculator. All you need to know is the room area, height, and CFM.

This is the formula for ACH (air changes per hour):

**ACH = CFM x 60 / (Area x Height)**

where ‘Area’ is the area of the space where you intend to have an HVAC device and ‘Height’ is the height of the ceiling.

The formula is basically ‘how many cubic feet of air can an HVAC unit provide every hour’ divided by the volume of the room.

We always get CFM but that is the volume of air *per minute*. In order to calculate air changes * per hour*, we have to translate that into hours. Hence the multiplication with 60 in the equation above.

The volume of a room is simply calculated by* length*width*height*. Obviously, the multiplying length of the room with its width will give us the surface area (‘Area’). To get the volume, we have to multiply the area by height.

If you have any questions regarding the calculation of air changes per hour, you are welcome to ask us in the comments below.

What would be a simple solution for a dental office for covid air change

Hello Paul, HVAC devices that purify the air are known as air purifiers. They usually have the highest ACH (changes air 4-5 times per hour, if correctly sized).

You need a PCO quality air purifier that uses uv light as well as reactor pads that go to .01 micron. Don’t bother with carbon or mere filters as they do not kill covid….

I’m installing an extractor fan in my dental office which works at (Airflow GX9) 728m³/h or 202L/s

My room volume is 29m3

What is the ACH?

How quickly will my fallow time of 1 hour be reduced?

I’m sorry if I’m not getting this

Thanks again

D

Hello Derek, here you take two figures into account; 728 m³/h airflow and 29 m³ room volume. In 1 hour, the fan pushes out 728 m³ of air. Given the room volume is 29m³, that means it the air will be changes 728/29 = 25.1 times every hour. In short, ACH is 25.1. That’s a very high ACH. Average HVAC devices range from 2 to 8 ACH. The 728m³ device might be an overkill for a 29 m³ room. Hope this helps.

Perfect thank you for your help.

We have to reduce the time between stopping generating an aerosol and leaving fallow time. The quicker I can recirculate the air the sooner it is safe to resume seeing the next patient. UK has 60 minutes. Evidence is ACH reduces 63%

If adding an air purifier to supplement current ventilation system, would you add the two ACH together for total ACH?

Hello Sara, a very interesting question. In the most basic sense, we can add the airflow of both air purifier and ventilation system; the air volume of the room remains the same. Theoretically speaking, we can add the two ACH together, yes. However, the function of the ventilation system and air purifier is not the same; ventilation circulates the air while the air purifier gathers the airborne particles.

I’m missing something. Shouldn’t the formula be:

ACH = CFM x 60 / Area x Height)

Hello Susan, you’re right, we’ve added that missing ’60’ into the equation, thank you for the heads up.

Is this considering both supply and exhaust? So the formula in sense is (total cfm*60)/volume of space?

Hello Mark, ACH of HVAC devices such as air purifiers is usually given for supply, and it’s based on the airflow. Airflow is measured in CFM (cubic feet per minute). The reason why we need volume of space in the equation is because we’re filling a certain volume with that airflow. 60 is because we have to convert from CFM to CFH (so, from cubic feet per minute to cubic feet per hour). That’s because ACH is usually given on per hour basis.

Hi, so is it possible for a UVC air purifier to be effective in a space together with an HVAC system in a big office where the air is being recirculated between rooms in a big building?

If so, if the HVAC is re-circulating the air around the building at say 2 ACH and the air purifier is rated to perform 8 ACH, how do calculate the effectiveness of the air purifier if the HVAC system is circulating contaminated air into the room whilst the air purifier is trying to do its job?

Hello John, UVC air filter will be effective at purifying the air. However, you have to be careful about air purifier capacity. If, for example, the air purifier has a maximum airflow of 200 CFM, it will run at 200 CFM.

What needs to be taken into consideration is that ACH is based on two factors: air purifier airflow and volume of air. Air purifier airflow is constant; however, the volume of air is not. By recirculating air between rooms in a big building, you’re effectively expanding that volume quite a bit. Effectively, an air purifier that has the capability to achieve 8 ACH in 1 room, will achieve 1 ACH if it has to purify the air in 8 rooms (same volume rooms).

The exact calculation of ACH when the circulation is involved start gaining in complexity quite a bit.

when calculating air / space volume do you include the air space above the ceiling if it is plenum return? It seems like you would since this air is part of the system and needs to be diluted also.

Your thoughts?

Hello Mike, for a thorough ACH calculation, the whole space volume needs to be included; since all the air need in the room needs to be changed. If the plenum return created additional volume, you have to count that into the calculation.

Can one help what to keep the ideal CFM value of a room…i am actually worling now on a IT building hvac systems :it has conference room,server room,meeting room,training room, ODC s etc…for each of these rooms what can be the suitable CFM value?

Hello Arya (love the name, go team Stark!), specific room such as conference rooms, server rooms, and so on have different recommended CFM values. For example, server rooms have CFM 3-4 or something like that. HVAC experts usually have that list; it would be best to consult them about every room. You can reach them free of charge here; they are best equipped to help you out.