How to calculate airchanges per hour in a room

A cleanroom is a room where there is control over the concentration of airborne particles. Other factors are also controlled, including temperature, humidity and air pressure. According to international authority on cleanrooms, William Whyte, air changes per hour is equal to the air volume rate, expressed in cubic metres per hour, divided by the volume of the room in cubic metres.

Add together the total volume of air being supplied to the room by the air conditioning units or other HVAC systems. This information will normally be available in the manuals that come with the units. You will need a figure in cubic metres per hour. This figure is the air volume rate.

Measure the length of the room in metres. Measure the width of the room likewise. Measure the height of the room, utilising ladders if necessary. Multiply the length by the width by the height to arrive at a figure representing the volume of the room in cubic metres.

Divide the air volume rate by the room volume. This will give you the air changes per hour for the room. Compare this with recommended air changes per hour for given rooms, as provided by a reputable body like The University of Liverpool’s Safety Advisory Service. (Please see Ref. 2).


Take precautions when climbing ladders to ensure ladder stability.

Things You'll Need

  • Calculator
  • Tape measure
  • Ladders
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About the Author

Frank Luger had his first educational resources published in the early 1990s. He worked on a major reading system for Cambridge University Press, became an information-technology adviser and authored interactive whiteboard resources for "The Guardian." Luger studied English literature and holds a Bachelor of Education honors degree from Leeds University.