HVAC ENGINEERING TOOL

Air Changes per Hour Calculator

Calculate volumetric ACH or required airflow from clear internal room dimensions.

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Formula / Method

ACH = airflow (m³/h) ÷ clear internal room volume (m³)

Engineering Method & Assumptions

Room volume uses clear internal length, width and height.
Metric and imperial dimensions convert through SI without intermediate rounding.

Applicability: Volumetric ACH alone does not establish outdoor-air ventilation or code compliance.

WHEN TO USE THIS TOOL

Useful during airside & ventilation work.

Quick-check airflow, velocity, room-air-change or ventilation relationships during design reviews.
Compare alternate design conditions before moving into detailed equipment or duct selection.
Use as a transparent check alongside project criteria, applicable standards and manufacturer data.

Frequently asked questions

What does ACH mean?

ACH is the number of room-volume air changes represented by volumetric airflow each hour.

Is ACH the same as fresh-air ventilation?

No. Total volumetric airflow may include recirculated air and does not identify the outdoor-air fraction.

Should room volume use clear internal dimensions?

Yes. Use the clear internal length, width and height of the space being assessed.