Air Changes Per Hour (ACH) Calculator
The Air Changes Per Hour (ACH) Calculator tells you how many times the entire volume of air in a room is replaced each hour by your ventilation. It uses the airflow rate and the room volume: ACH = (airflow in CFM × 60) ÷ room volume in cubic feet.
Formula
ACH = (Airflow CFM × 60) ÷ Room Volume (cubic feet)
- One air change means a volume of air equal to the whole room has been supplied or removed.
- Formula: ACH = (CFM × 60) ÷ room volume in cubic feet (the 60 converts per-minute airflow to per-hour).
- Room volume = length × width × ceiling height, all in feet.
- Typical targets vary by use: living spaces often aim for 0.35–1 ACH, while kitchens, bathrooms and labs need much higher rates.
Example Calculation
Inputs
- Airflow (CFM): 200
- Room Volume (cubic feet): 1600
With 200 CFM of airflow into a 1,600 cubic-foot room: ACH = (200 × 60) ÷ 1,600 = 7.5 air changes per hour.
Frequently asked questions
What is air changes per hour?
ACH is how many times the full volume of air in a space is replaced in one hour by the ventilation system.
How do I find room volume?
Multiply length × width × ceiling height in feet to get cubic feet.
What is a good ACH value?
It depends on the room. Bedrooms and offices often target under 1 ACH, while kitchens, bathrooms and laboratories require many more.
Why multiply by 60?
Airflow in CFM is per minute, so multiplying by 60 converts it to cubic feet per hour before dividing by the room volume.
Can I use metric units?
This version uses CFM and cubic feet. For metric, convert m³/h and m³, where ACH = airflow (m³/h) ÷ volume (m³).