HVAC ACH Calculator Explained: Formula, Examples & Ventilation Guide
Air Changes per Hour (ACH) is one of the most important parameters in HVAC ventilation design. It indicates how many times the total volume of air inside a room is replaced with fresh or conditioned air in one hour. A properly designed ACH ensures good indoor air quality (IAQ), occupant comfort, odor control, and compliance with ventilation standards such as ASHRAE 62.1.
Whether you are designing an office, hospital, laboratory, restaurant, or residential building, understanding ACH helps ensure the HVAC system supplies adequate ventilation while maintaining energy efficiency.
What is Air Changes Per Hour (ACH)?
ACH is the number of times the total air volume inside a room is replaced within one hour. The higher the ACH value, the more frequently the air is refreshed.
| Term | Description |
|---|---|
| ACH | Air Changes Per Hour |
| CFM | Cubic Feet per Minute |
| Room Volume | Length × Width × Height |
ACH Formula
ACH = (CFM × 60) ÷ Room Volume (ft³)
Where:
- ACH = Air Changes per Hour
- CFM = Airflow supplied to the room
- Room Volume = Length × Width × Height
How to Calculate ACH
- Measure the room length, width, and height.
- Calculate the room volume.
- Determine the supply airflow in CFM.
- Multiply CFM by 60.
- Divide by the room volume.
Example Calculation
Room Size = 20 ft × 15 ft × 10 ft
Room Volume = 20 × 15 × 10 = 3000 ft³
Airflow = 600 CFM
ACH = (600 × 60) ÷ 3000 = 12 ACH
Always verify that the calculated ACH meets the applicable building code, ASHRAE recommendations, and project specifications. Excessively high ACH increases energy use, while low ACH may reduce indoor air quality.
Typical Recommended ACH Values
| Application | Typical ACH |
|---|---|
| Living Room | 4–6 |
| Bedroom | 2–4 |
| Office | 4–6 |
| Classroom | 4–6 |
| Restaurant Dining Area | 6–10 |
| Hospital General Ward | 6–12 |
| Operating Room | 15–25 (project and healthcare standards may vary) |
| Laboratory | 6–12 |
