How to Size an Air Scrubber Using Air Changes Per Hour

Choosing an air scrubber is not guesswork. It comes down to a simple calculation based on the size of the room and how many times per hour you want to filter all the air in it. Once you know the method, sizing any space takes a minute.

The Key Concept: Air Changes Per Hour

Air changes per hour, or ACH, is how many times the total volume of air in a room passes through the filter in an hour. Remediation work commonly targets a high number of air changes, often around 4 to 6 ACH or more depending on the job, to keep airborne contamination under control.

The Calculation

1. Find the room volume: length times width times ceiling height, in cubic feet
2. Multiply the volume by the target air changes per hour
3. Divide by 60 to get the required airflow in cubic feet per minute (CFM)
4. Choose an air scrubber rated at or above that CFM

For example, a 2,000 cubic foot room at 6 ACH needs 12,000 cubic feet filtered per hour, or about 200 CFM. Pick a unit that delivers at least that with the HEPA filter installed.

Rate at the Filter, Not Free Air

Machines are often rated at free airflow and at filtered airflow. The number that matters for sizing is the airflow with the HEPA filter in place, since that is how the machine runs on the job. Use the filtered CFM, and remember airflow drops as filters load with dust.

When to Use Multiple Units

• Large or open spaces may need more than one unit to hit the target ACH
• Running multiple smaller units can improve air distribution in odd shaped rooms
• One unit can scrub while another runs as negative air for containment

Build in a Margin

Because airflow falls as filters clog, size with a little headroom rather than exactly at the minimum. That keeps you at your target air changes even as the filter loads. Browse our HEPA air scrubbers with their rated airflow so you can size correctly.