Air Changes per Hour Calculator
Formula: ACH = (CFM × 60) ÷ (Length × Width × Height)
What is Air Change per Hour (ACH)?
Air Changes per Hour (ACH) is a key ventilation metric that measures how many times the air within a defined space (like a room or building) is completely replaced with fresh air in one hour. It’s essential for indoor air quality, especially in environments such as hospitals, laboratories, schools, cleanrooms, and homes.
The ACH Calculator allows you to quickly estimate the ventilation rate of a space based on its size and airflow rate.
Why is ACH Important?
ACH is critical for:
- Ensuring healthy indoor air quality
- Reducing airborne contaminants, allergens, and pathogens
- Maintaining cleanroom or lab conditions
- Providing proper ventilation in occupied spaces
- Meeting HVAC and building code standards
Air Change per Hour (ACH) Formula
The standard formula to calculate ACH is:
CFM X 60 / Room Volume
Where:
- CFM = Airflow in cubic feet per minute
- Room Volume = Length × Width × Height (in cubic feet)
- 60 = Number of minutes in an hour
You can also calculate using m³/h and room volume in cubic meters for metric systems.
Recommended ACH Values (General Guidelines)
| Environment | Recommended ACH |
|---|---|
| Hospital Operating Room | 15–25 ACH |
| Classroom | 4–6 ACH |
| Office Space | 2–4 ACH |
| Laboratory | 6–12 ACH |
| Residential Home | 0.35–1 ACH |
Note: Specific standards may vary by region and regulatory body (e.g., ASHRAE, CDC).
How to Improve ACH
To increase ACH and improve ventilation:
- 🌀 Upgrade HVAC systems or fans
- 🧰 Add portable air purifiers or exhaust fans
- 🚪 Open windows or vents (where feasible)
- 🛠️ Use air exchange systems in sealed rooms
