How to Calculate Air Changes Per Hour (ACH) for Barns and Warehouses

Air changes per hour (ACH) is a simple, useful way to measure how well a building is ventilated. It tells you how many times the entire volume of air in a space is replaced in one hour. The higher the ACH, the fresher the air.

The Basic Formula

To estimate ACH, you need two numbers: the airflow in cubic feet per minute (CFM) and the volume of your space in cubic feet. First, find your space volume: length × width × height. Then use: ACH = (CFM × 60) ÷ space volume.

What’s a Good ACH?

The right target depends on the type of building and what’s inside it. Livestock barns, poultry houses, and busy workshops generally benefit from higher ventilation rates than quiet storage spaces. The goal is to remove heat, humidity, and stale air fast enough to keep the environment healthy and comfortable.

How HVLS Fans Help

HVLS fans move very large volumes of air — the REM-4 is manufacturer-rated up to 77,300 CFM — which makes them a powerful tool for improving air movement and supporting healthy ventilation in big, open buildings.