Air Changes per Hour (ACH)

All construction tools

Measure ventilation effectiveness by calculating room air replacement rates.

Room & Airflow Data

Measurement System

Length (ft)

Width (ft)

Height (ft)

Airflow Rate (CFM)

* Airflow is usually measured in CFM (Cubic Feet per Minute) for fans and HVAC units.

Ventilation Results

Check your ventilation?

Enter the room dimensions and the airflow rate of your HVAC or purifier to calculate the ACH.

About this calculator

Overview

Air Changes per Hour (ACH) is a measure of how many times the total volume of air in a defined space is replaced with new air every hour. In a perfectly mixed space, 1 ACH means 63.2% of the original air has been removed. Higher ACH values are required for sensitive environments like hospitals (6-12 ACH) or laboratories, while residential homes typically aim for 0.35 to 2 ACH.

💡

Pro Tips

For standard residential rooms, aim for 2-4 ACH for good air quality.
If you're using an air purifier, ensure it achieves 4-5 ACH for the specific room size to effectively remove allergens.
Kitchens and bathrooms usually require much higher ACH when in use to remove moisture and odors.

Fun Facts

"In the 19th Century, Florence Nightingale advocated for high ACH in hospitals long before modern HVAC to reduce the spread of disease."
"A typical modern energy-efficient home might have an ACH as low as 0.1 without active ventilation, which can lead to poor indoor air quality."
"HEPA purifiers are often sized by their ACH rating for a specific square footage."

Formula: ACH = (Hourly Airflow Volume) / (Room Volume)