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Ventilation Air Changes Calculator

Enter the ventilation airflow and the room volume to get the air changes per hour.

How it is calculated

The tool divides the supply airflow by the room volume: ACH = Q / V, with Q in m³/h and V in m³. Two companion figures are given: the airflow in litres per second, L/s = Q / 3.6 (because 1 m³/h is 1000 L per 3600 s), and the time for one complete air change, t = 60 / ACH in minutes. The room volume is length × width × height, which you work out yourself in m³. Zero or negative inputs give no result. The model treats the air as perfectly mixed and uses the airflow you enter, so it is not a measurement of real leakage or of how well the air reaches every corner. It does not apply any code requirement; compare the ACH with the figure that your local rules or design specify for the room type.

Worked example

A fan rated at 150 m³/h in a room of 60 m³ (for example 5 m × 4 m × 3 m). ACH = 150 / 60 = 2.50 ACH. In litres per second: 150 / 3.6 = 41.7 L/s. Time for one air change = 60 / 2.5 = 24.0 min. The whole volume of air is therefore replaced two and a half times an hour, or once every 24 minutes.

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Common mistakes

  • Entering the airflow in L/s or CFM. The field is m³/h, so convert first, for example 40 L/s is 144 m³/h.
  • Entering the room floor area in m² instead of the volume in m³. Multiply by the ceiling height.
  • Using the fan's free-air rating when it is connected to ducting. Real airflow through ducts and grilles is lower than the catalogue figure, which pushes the true ACH down.
  • Dividing the combined airflow of several rooms by the volume of just one room, or the reverse.
  • Treating ACH as the time between changes. The minutes figure is a separate value, and it is the reciprocal: higher ACH means fewer minutes.

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.

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