Ventilation Air Changes Calculator

About this calculator

Air changes per hour (ACH) measures how many times a room's full volume of air is replaced with fresh air in one hour, calculated as ACH = ventilation airflow (m³/h) ÷ room volume (m³). Building codes and ventilation standards specify minimum ACH targets per room type — bathrooms, kitchens and commercial spaces typically require higher ACH than bedrooms or offices, because they generate more moisture, odors or airborne contaminants that need to be cleared.

HVAC engineers, builders and building inspectors calculate ACH to verify that an installed ventilation system actually meets code or design requirements for a given space, because airflow rating alone (m³/h) is meaningless without knowing the room volume it has to clear — the same fan that ventilates a small bathroom adequately would be badly undersized for a large open-plan kitchen.

This calculator takes your ventilation system's airflow and the room's volume and returns the resulting air changes per hour, so you can check it against the recommended ACH for that room type.