Fan Selection Concepts: Air Changes Per Hour

Calculate your Fan Requirements by Considering the Air Changes Per Hour Required

air changes per hour
air changes per hour

When specifying a fan or ventilation system, one of the most important considerations is the air changes per hour. Quite simply this is the number of changes in the air (also known as air change rate) needed to sufficiently ventilate the room. It is equally crucial to take into account the volume of outside air (fresh supply air) that the room requires. This can be calculated based on the number of people typically living or working in the space and the size of the room itself. These requirements can depend on a number of factors but the nature of the room's usage is a key place to start. It is generally considered that four air changes per hour is the minimum air change rate for any commercial or industrial building but with many environments been more sensitive and occupied more, higher air change rates apply.


It is important to note that these figures are only for general reference and do not include specific situations where contaminated air, chemical dust or gas is in the air stream such as local exhaust ventilation, cleanrooms, powder control and other stringent applications. 

What is ACH?

Air changes per hour is a measurement of air volume that is added to (or removed from) a room divided by the total volume/size of the room. Put simply, it measures how many times the air in the room is replaced. Higher ACPH values mean better ventilation. The formula is as follows:

ACPH = Q / Vol


  • Q = Volumetric flow rate of air/Amount of air in cubic metres per hour (m³/h)
  • Vol = Volume /Size of Room: L × W × H in cubic metres (m³)

How to Calculate the Volume of Air

To find the volume of air required to properly ventilate a space, you'll need to have an estimate of the typical number of people occupying it and know what the space will be used for. Then, simply use the table below to multiply the number of people by the required m3/hr per person to determine the required volume of air. In unoccupied spaces such as public garages you do not need to know the number of occupants and can refer to a common number of air changes required for that environment.

Airflow is the volume of air moved by a fan per unit of time, usually expressed in cubic metres per hour M3/hr

The table below outlines reference values of air volume split by room usage for commonly occupied spaces according to DIN 1946 part 2. This is a standard set by the German Institute for Standardisation and is accepted worldwide.



Minimum Outside Air Stream per Person  (m³/h)  Recommended Number of Air Changes per Hr
Toilets 30    4 (private) 10 (public)
Showers    60    6 (private) 10 (public)
Offices   40-60    6
Restaurants     50    12
Canteens     30     12
Classrooms     30    5
Conference Facilities     30    8
Multi-purpose Halls     30    5
Gyms     30    5
Waiting Rooms     30 6

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