Air Changes Per Hour Calculator

Air changes per hour is airflow measured against the size of the room, so a fan that suits a bedroom is nowhere near enough for a commercial kitchen. Enter the room dimensions with the CFM a fan or register actually delivers and this returns the rate being achieved, the minutes it takes to replace the air once, and how far that sits from the design rate for the room type. Run it the other way to get the airflow a target rate needs. A third action reads a blower door result as ACH50 against a code limit and divides it down to a rough natural rate.

Calculator Type
Room Measured In
The measured or rated cubic feet per minute the fan or register delivers.
Leave it blank for the usual 8 ft, which is 2.44 m.
These are the usual design rates. Put your own figure in the box below to ignore them.
Leave at zero to use the rate shown against the room type.
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Room Measured In
Leave it blank for the usual 8 ft, which is 2.44 m.
These are the usual design rates. Put your own figure in the box below to ignore them.
Leave at zero to use the rate shown against the room type.
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Area and Height In
Cubic feet per minute at 50 pascals, straight off the manometer.
Leave it blank for the usual 8 ft, which is 2.44 m.
The rule of thumb divides ACH50 by about 20. Sheltered, single storey and mild climates sit nearer 25, exposed multi-storey ones nearer 14.
The 2021 IECC asks for 3.0 in climate zones 3 to 8 and 5.0 in zones 1 and 2. Older code and existing houses are looser.
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