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Air Changes per Hour Calculator
Find out how many times per hour the air in a room is fully replaced, based on room size and your ventilation system’s airflow rate.
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What is Air Changes per Hour (ACH)?
Air changes per hour tells you how many times the entire volume of air in a room is replaced with fresh or filtered air in one hour. It’s a common benchmark for ventilation design, HVAC sizing, and indoor air quality planning.
The Formula
ACH is calculated as ACH = (Airflow in CFM × 60) ÷ Room Volume in cubic feet. Room volume is simply length × width × height. A higher ACH means air is being refreshed more frequently.
- Typical office space: 4-6 ACH
- Residential rooms: 2-4 ACH
- Hospital operating rooms: 15-25 ACH
This calculator is for general planning and educational purposes — always check local building codes and mechanical engineering guidance for spaces with specific ventilation requirements.
Last reviewed August 2026