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Transfer Grille Size Calculator
Adjust the data in the blue boxes, press enter or click "Calculate"
  Door Undercut  
Room CFM   Height   Width   Needed Transfer Grille Area Jump Duct Diameter
  " by " 24.5 inches square (0.2 sq. ft.) 5.6 inches
If the free area of grille is % , and side (a) is " , side (b) should be 3.71 inches  
The calculated grille area will be large enough to maintain less than 0.012" wc (3 pascals) pressure difference to prevent the pressurization of bedrooms and depressurization of the common area. This limits the amount of air that will be forced into or drawn from the attic, crawlspace etc. More info on transfer grilles from this link;
The Building Science Corporation.

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