Air Changes Per Hour: What It Is, How To Calculate ACH?

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:

ACH Calculator

0.00 ACH

Air Changes Per Hour

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. 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 calculated by length*width*height. Multiplying the 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. 

How Many ACH Different Air Purifier Brands Use

The calculation of the recommended coverage area in the air purifier specification is based on CADR rating, maximum airflow, and ACH.

Essentially, to calculate the recommended coverage area, different air purifier companies use 1-5 air changes per hour. The ones that use 5 ACH are very thorough when removing air pollutants that recommended room size, using 2 ACH less so.

Here is a list of how many ACH different air purifier brands usually use to calculate the recommended coverage area:

  • Alen BreatheSmart uses 2 ACH. Example: Alen BreatheSmart 75i – #1 ranked air purifier – has a 1,300 sq ft recommended coverage area. Its maximum airflow is 350 CFM. At 5 ACH, the recommended coverage area is 520 sq ft.
  • Coway air purifiers’ coverage area is based on either 2 ACH or 5 ACH. Example: Big Airmega 400 has a 1,560 sq ft coverage area with a 350 CADR rating (2 ACH). The high-performance Coway AP-1512HH has a 361 sq ft coverage area with a 246 CADR rating (5 ACH).
  • Molekule has a recommended coverage area but provides no ACH, CADR, or maximum airflow data. Molekule Air, for example, has a 600 sq ft coverage area, but it’s impossible to deduce how many air changes per hour does it make.
  • Honeywell uses 5 ACH. Example: Honeywell HPA300 has a 465 sq ft coverage area with a 300 CADR rating (5 ACH).
  • Levoit air purifiers are interesting; they use 3.33 ACH with their best model. Example: Levoit LV-H135 has a 463 sq ft coverage area and 360 CADR rating. Air is changed every 18 minutes; thus, the Levoit unit makes 3.33 air changes per hour.
  • Okaysou uses 3 air changes per hour. Example: Their most popular Okaysou AirMax8L air purifier has a 500 sq ft coverage area, with a 210 CADR rating (3 ACH).
  • Dyson is very shy about revealing the room sizes. That’s why it’s not possible to calculate the ACH for any Dyson air purifier

Of all the HVAC devices, air purifiers are unique as far as ACH is considered because their job is the most closely rated to the ACH specification. In essence, ACH is the second-best determining factor that indicates how well air purifiers clean the air. 

It is important to understand that the calculation of ACH is solemnly based on airflow. It is not a measure of how well the air purifier’s filtration system works; it does not measure the effectiveness of HEPA filters, activated carbon filters, or even ozone generator filters. High ACH doesn’t, for example, directly reduce the chances of mold growth (mold inspection and testing can attest to that).

There is another better determining specification valid for air purifiers that measures the effectiveness of the filtration system; the CADR rating. The CADR rating is proportional to both ACH as well as various filters air purifier might employ. Because of this, ACH calculation and the consequent CADR calculation are most appropriate for air purifiers. 

To calculate the room size based on airflow (in CFM), you should use the CFM calculator here.

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

48 thoughts on “Air Changes Per Hour: What It Is, How To Calculate ACH?”

    • 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….

  1. 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

    • 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.

  2. 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%

    • 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.

    • 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.

  3. 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.

  4. 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.

  5. 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.

  6. If a room only has a window that can be opened with no HVAC or fan how many ACH would you achieve, in my simple mind it will be too inconsistent to measure as it would rely on the outside air movement, what doors and windows are open in other rooms in the building etc.

    • Hello Andy, theoretically ACH is calculated with the use of CFM output and the total volume of air being replaced. Practically, however, we can’t 100% determine the volume of air that needs to be replaced because of the doors, windows, and independent air movement.

  7. hello’
    i have cfm on supply air duct and return air duct(for exhaust fan), how to calculate ACH from these. which cfm need to consider (Supply or return).

  8. Hello,
    We have a 24,000 square foot warehouse with 23 feet clear height based is Las Vegas, Nevada.
    How many Phoenix model 2231 swamp coolers would I need with 5hp motors? And what would the ACH be?
    By my calculations, I would need 14 swamp coolers.
    24,000 x 23 = 55200 / 2 = 276,000 / 20,000 cfm swamp = 13.8 (14) swamp coolers needed.
    If my calculations are correct, how many ACH’s would we be getting?

    • Hello Brian, the total volume of air is 552,000 ft3. For 1 ACH, you would need 552,000/60 = 9,200 CFM. What is the maximum CFM of 1 Phonenix model 2231 swam cooler? We can calculate from there on. For example, it has 920 CFM, you would need 10 of them for 1 ACH. For 2 ACH, you would need 20 and so on.

  9. Hello,
    I have a two level museum. On the museum first floor I have 4 fan coils (Mitsubishi PEFY-P54) spread-out through the open space. The total fan for each f/c is 800 CFM. To calculate the ACH would the formula be 4*800*60 / Area * Height.

    • Hello Dan, fans, in general, just push the air around. As such it’s hard to speak about air changes per hour; the addition of CFM doesn’t fully apply. ACH calculation is most appropriate for air purifiers.

  10. good morning,

    using this formula ACH = CFM x 60 / Area x Height), what can I add into that formula to calculate maintaining a certain temperature . example 30 degrees above ambient inside of a structure.

    • Hello Gabe, as defined, ACH is not a function of temperature. The one thing that changes significantly as a function of temperature is the density of air, and thus the volume of air. The volume of the room is fixed; the temperature of the air can influence the maximum CFM of HVAC devices. Maybe you can use that and implement the temperature in the equation in some way.

  11. Thanks Learn Metrics !

    The formula has been very helpful. I see a lot of fellow Dentist concerned with the current fallow break after AGP. We did a lot of research into solutions that would work for us, and i would like to share to hopefully save people being mislead or buying something that isn’t working as expected.

    What we found was that most filters don’t do what they claim. Lots of these UV filters need to be exposed to the virus/bacteria for an extended time (around 13 seconds) from what we seen, and most of these portable units on sales now push the air through in a split sec, not allowing enough time for the UV to break down the viruses etc.
    Many of the filters claiming to have HEPA filter grade filtration don’t meet the standard of 99% of particles captured at 0.3 micron, and are allowing thousands of particles through.

    In the end we came across Healthyway air purifiers which filters 100% 0.3 micron particles and 99.97% of 0.007 micron particles, ie it filters and breaks down smaller particles than Covid19 which is around 0.125 microns. Loads of videos on youtube showing zero particles getting through the filter, which are interesting.

    Ended up ordering 4 “Castlewellan” Air purifiers (1 for each treatment room and 1 for waiting area) from a company called OKTOair (which are the UK/Ire distributors for the Healthway Air purifiers), units took a couple of weeks to come as they were sold out of the size we wanted and had incoming stock.

    Have to say, im very pleased with the units, at they well built, reasonably priced and after watching the videos on youtube, confident in there health benefits for our clients and staff.

    • Hi Michael!

      I can’t find any independent lab test results for Healthyway. This industry is not properly regulated and any company can say whatever they want to. If they want credibility, they should openly share their test results.

    • Hello Mary, yes. ACH calculation depends on two main factors, CFM of an HVAC device and the size of the room. If you change either of these things, the ACH will change as well.

  12. Hello dear

    Thank you for your explain

    I have lab size 175 SQF and ceiling hight 6 feet ,so what is the CFM air flow need to extract air 6 time every hour

    Thank you

    • Hello Hamza, in short, you need 6 ACH airflow. The volume of the air is 175 sq ft * 6 ft = 1050 cubic feet. We need to change that air 6 times per hour, so the total volume we’re looking at is 6300 cubic feet. In terms of airflow (measured in cubic feet per minute or CFM), we need to proceed with this calculation: 6300 cubic feet / 60 minutes = 105 CFM. You need 105 CFM airflow for your lab. Hope it helps.

  13. Hi, I am using the winix zero pro AZPU370-IWB) with a CADR smoke/dust/pollen 258/264/301. The use in is a dental surgery with vapor aerosol potentially containing Covid-19. Assuming the aerosol particle size is the same as smoke (the smaller of the 3) the CADR would be the lower 258 rating. The size of the surgery is below;

    Length = 14 ft
    width = 9ft
    height = 9ft

    When put into the online calculator above, the ACH is approx 14. My only question is as mentioned on this site, all Winix purifiers use the 5ACH AHAM calculation. what is this exactly? and what does that mean? will the purifier only achieve a max of 5ACH at max speed in a recommended size room? of will it achieve the full 14ACH in the real world in the size of the room above?

    this relates the UK dental guidance in relation to ACH and time allowed between patients. 1-5ACH 30 min down time, 5-9ACH 20 min down time, 10+ACH 10 min down time. All with the use of High volume aspiration which is which is supposed to remove approx 90% of aerosol anyway.

    Hope that makes sense.

    • Hello Gavin, thank you for the insight. Winix usually has the recommended coverage area calculated, based on 5 ACH. The full volume of dentistry surgery is 1134 cubic feet. To calculate how much airflow in (CFM) you would need for a certain number of ACH, you can use this equation:

      CFM = (Number of ACH) * 1134 ft3 / 60

      Let’s input ACH = 5. We get 94.5 CFM. Air purifiers can have above 400 CFM airflow.
      If we input ACH = 14, we get 264.6 CFM. According to your calculation, the Winix unit you have probably has this airflow.

      Be mindful, however, that ACH is a measure of air circulation, not air purification. CADR numbers are unrelated to ACH. Hope this makes sense, if not, just ask us here and we’ll try to solve the issue.

      • Hi thank you that makes perfect sense. Yea ACH >/10 is what is needed. Not air purification. So on that the machine does the job needed in that size room. Thank you for your help

  14. Hi!

    How do I calculate for the required outdoor air using ACH if the system that already built has split type wall mounted FCU and exhaust grilles but no supply of outdoor air?

    • Hello there, ACH is a theoretical calculation, based on the airflow of a HVAC device. Practically, in your case, with no supply of air from outside, the air is not exactly exchanged with the new outdoor air. If you could elaborate a bit more about your question, we might be more helpful.

  15. I am trying to calculate # of room changes, but have two separate ventilation sources. Do I add these two values together or should I average them?
    Vent. #1 874 cfm and Vent. #2 993 cfm

  16. How can I select ACH for Packing House (washing and sorting line area)my Area is about 1422475 cubic ft. Is their any code and standard for this?
    kindly help in this regards

    • Hello there, you can follow the EPA recommendations for ACH for different kinds buildings. The closest thing to ‘packing house’ would be ‘Assembly halls’ with the recommended ACH of 6-8.

      To calculate CFM you need, you can use the following formula: CFM = (Area x Height x ACH) / 60.
      If you have 1.422.475 cubic ft of space (this is Area x Height).
      If you insert 6 ACH, you get a total of 142.247,5 CFM. That’s quite a lot. If you use ACH 8, you will get even more. Hope this helps.

  17. I was wondering if 2 or 3 air purifiers of 125 cfm each are placed in a room of 12x12x8, how do I calculate the ACH or the time to clean the room with those air purifiers.

    • Hello Ben, sure thing. 2 air purifiers = 250 CFM combined, 3 air purifiers = 375 CFM combined. It is a small 144 sq ft room; with 2 air conditioner you achieve 13 ACH, with 3 air purifiers you achieve about 19.5 CFM (the complete air volume of the room is changed every 3 minutes or so).

  18. I have a 25,000 law firm and approximately 76 offices with furniture throughout. With approximatley 40 full time employees, how do account for partitions, columns, people etc when determining total volume of air to be exchanged?

    • Hello Tom, usually the volume of furniture, people, even plants and so on, is not accounted for when calculating the total volume of space. It does make for a less accurate estimate, but in HVAC world this is considered to be acceptable; the volume of all that stuff is usually not sufficiently significant.


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