What are Air Changes per Hour (ACH)?
Air Changes per Hour (ACH) is a measure of how many times the total volume of air in a room or space is replaced with new, fresh air in one hour. It is a critical metric for assessing ventilation effectiveness and ensuring good indoor air quality.
The calculation is straightforward: ACH = (Airflow Rate in CFM × 60) / Room Volume in Cubic Feet. A higher ACH value indicates better ventilation, which is crucial for diluting and removing indoor pollutants, allergens, and airborne pathogens.
Recommended ACH for Different Spaces
| Space Type | Recommended ACH |
|---|---|
| Residential Home (General) | 0.35 - 1.0 |
| Office | 4 - 6 |
| Classroom | 3 - 4 |
| Hospital Patient Room | 6 - 8 |
| Hospital Isolation Room | 12+ (Negative Pressure) |
How to Use the Calculator
- First, calculate the volume of your room (Length × Width × Height) and enter it in cubic feet.
- Find the airflow rate of your ventilation system, air purifier, or fan. This is usually specified in Cubic Feet per Minute (CFM).
- Enter both values into the calculator.
- Click "Calculate ACH" to see how many times the air in your room is fully replaced each hour.
Frequently Asked Questions
How do I find the CFM of my device?
The CFM rating is usually listed on the product's specification sheet, in the user manual, or on a label on the device itself. For central HVAC systems, this information may be in the technical documentation or can be provided by your HVAC technician.
Is a higher ACH always better?
While a higher ACH generally means better ventilation, extremely high rates can lead to drafts, noise, and increased energy costs for heating or cooling the incoming air. The goal is to meet the recommended ACH for the specific space and its use, providing a balance between air quality and comfort.