\[ACH = \frac{Q}{V}\]
Variables
ACHair exchange rate (1/h)
Qventilation airflow rate (m³/h)
Vroom or building volume (m³)
Description
What is this formula?
This formula calculates the air exchange rate, also known as Air Changes per Hour (ACH), which indicates how many times the air inside a space is replaced in one hour.
When to use it
Use it when evaluating indoor air quality, HVAC system performance, contaminant control, or ventilation requirements in buildings.
Example
Data:
Q = 2400 m³/h
V = 800 m³
Formula:
ACH = Q / V
Substitution:
ACH = 2400 / 800
Result:
ACH = 3 h⁻¹
Applications
Building ventilation design, indoor air quality assessment, laboratories, hospitals, schools, industrial facilities, and clean rooms.
Note
This formula assumes perfect mixing of air within the space. Actual ventilation effectiveness may differ due to airflow patterns, short-circuiting, dead zones, and occupant distribution.