In HVAC, and especially with air purifiers, the ACH is an essential metric. ACH or ACPH stands for Air Changes Per Hour; for air purifiers are recommended to achieve 2-5 ACH in the space they are located in.
To help you out, we have created an online ACH calculator. It determines air changes per hour given the airflow (CFM) and space volume (width × length × height of the space). On top of that, we have also created a CFM to ACH chart for 100 sq ft rooms to 2,000 sq ft homes.
We will start with the calculator you can use yourself, explain how it works, and then we will look at the ACH formula, and the useful CFM to ACH conversion chart:
Alright, let’s look at one example to illustrate how this calculator works:
Say you have a 12×12 room with a standard 8 ft ceiling height. You get an AC or air purifier with 200 CFM airflow. How many air changes per hour (ACH) can you expect?
You simply input ’12’ in the width input, ’12’ in the length input, ‘8’ in the ceiling height slider, and ‘200’ in the CFM input field. Here is the result you get (screenshot):
We see immediately that a 200 CFM HVAC device in a 12×12 room (8 ft ceiling height) can achieve 10.42 ACH. That’s usually overkill for an air purifier; you would need anywhere from 2 ACH and up to 5 ACH (for people prone to allergies).
Let’s have a look at the formula we need to use to calculate the air changes per hour:
ACH Formula
1 ACH simply means that all the air in the room is vented through every full hour (60 minutes). The ACH calculation depends on two factors, namely the airflow from the HVAC device (expressed in CFM or Cubic Feet per Minute) and the volume of the air in the room. We can thus write this general ACH formula:
ACH = Airflow / Air Volume
Now, we express airflow in CFM (this is per minute; ACH is per hour). Air volume is simply calculated by multiplying space width × length × height (all dimensions should be in feet). Accounting for 1 hour = 60 minutes conversion, we get this simple ACH formula everybody can use:
ACH = CFM × 60 / (Width × Length × Height)
Namely, to calculate ACH / ACPH, we need to multiply CFMs by 60, and then divide this product by the product of width, length, and height (all in feet). Here is a quick example:
What is the ACH in a 16×20 ft room with a 9 ft ceiling height? The airflow is 300 CFM. We just insert all these numbers in the equation above like this:
ACH = 300 CFM × 60 / (16 ft × 20 ft × 9 ft) = 6.25 ACH
We calculated that the ACH in this room at 300 CFM is 6.25 ACH. You can use the calculator above to check this result.
We calculated ACH for spaces ranging from 100 sq ft to 2,000 sq ft at 100, 500, and 1,000 CFM airflow (standard 8 ft ceiling height) and summarized them in this neat ACH chart:
ACH Square Footage Chart
| Square Footage (Sq Ft): | Airflow (CFM): | Air Changes Per Hour (ACH): |
| 100 Sq Ft | 100 CFM | 7.50 ACH |
| 100 Sq Ft | 500 CFM | 37.50 ACH |
| 100 Sq Ft | 1,000 CFM | 75.00 ACH |
| 150 Sq Ft | 100 CFM | 12.50 ACH |
| 150 Sq Ft | 500 CFM | 25.00 ACH |
| 150 Sq Ft | 1,000 CFM | 50.00 ACH |
| 200 Sq Ft | 100 CFM | 3.75 ACH |
| 200 Sq Ft | 500 CFM | 18.75 ACH |
| 200 Sq Ft | 1,000 CFM | 37.50 ACH |
| 250 Sq Ft | 100 CFM | 3.00 ACH |
| 250 Sq Ft | 500 CFM | 15.00 ACH |
| 250 Sq Ft | 1,000 CFM | 30.00 ACH |
| 300 Sq Ft | 100 CFM | 2.50 ACH |
| 300 Sq Ft | 500 CFM | 12.50 ACH |
| 300 Sq Ft | 1,000 CFM | 25.00 ACH |
| 350 Sq Ft | 100 CFM | 1.67 ACH |
| 350 Sq Ft | 500 CFM | 8.33 ACH |
| 350 Sq Ft | 1,000 CFM | 16.67 ACH |
| 400 Sq Ft | 100 CFM | 1.88 ACH |
| 400 Sq Ft | 500 CFM | 9.38 ACH |
| 400 Sq Ft | 1,000 CFM | 18.75 ACH |
| 450 Sq Ft | 100 CFM | 1.67 ACH |
| 450 Sq Ft | 500 CFM | 8.33 ACH |
| 450 Sq Ft | 1,000 CFM | 16.67 ACH |
| 500 Sq Ft | 100 CFM | 1.50 ACH |
| 500 Sq Ft | 500 CFM | 7.50 ACH |
| 500 Sq Ft | 1,000 CFM | 15.00 ACH |
| 600 Sq Ft | 100 CFM | 1.25 ACH |
| 600 Sq Ft | 500 CFM | 6.25 ACH |
| 600 Sq Ft | 1,000 CFM | 12.50 ACH |
| 700 Sq Ft | 100 CFM | 1.07 ACH |
| 700 Sq Ft | 500 CFM | 5.36 ACH |
| 700 Sq Ft | 1,000 CFM | 10.71 ACH |
| 800 Sq Ft | 100 CFM | 0.94 ACH |
| 800 Sq Ft | 500 CFM | 4.69 ACH |
| 800 Sq Ft | 1,000 CFM | 9.38 ACH |
| 900 Sq Ft | 100 CFM | 0.83 ACH |
| 900 Sq Ft | 500 CFM | 4.17 ACH |
| 900 Sq Ft | 1,000 CFM | 8.33 ACH |
| 1,000 Sq Ft | 100 CFM | 0.75 ACH |
| 1,000 Sq Ft | 500 CFM | 3.75 ACH |
| 1,000 Sq Ft | 1,000 CFM | 7.50 ACH |
| 1,500 Sq Ft | 100 CFM | 0.50 ACH |
| 1,500 Sq Ft | 500 CFM | 2.50 ACH |
| 1,500 Sq Ft | 1,000 CFM | 5.00 ACH |
| 2,000 Sq Ft | 100 CFM | 0.38 ACH |
| 2,000 Sq Ft | 500 CFM | 1.88 ACH |
| 2,000 Sq Ft | 1,000 CFM | 3.75 ACH |
We see that the bigger the square footage we have, the lower ACH we get for the same airflow (CFM). We also see that if we increase CFMs (from 100 CFM to 500 CFM or 1,000 CFM), the ACH increases.
For example, if we have a 1000 sq ft home, we get:
- 0.75 ACH at low 100 CFM airflow.
- 3.75 ACH at medium 500 CFM airflow.
- 7.50 ACH at high 1,000 ACFM airflow.
We hope this chart, along with the calculator and formula above, help you to calculate the air changes per hour yourself. If you need a bit of help, you can always reach out to us via the comment section below. Give us a few numbers, and we can do some math together.