EfficientComfort.net
Transfer Grille Size Calculator
Enter the room CFM, undercut dimensions, free area and side (a), then click "Calculate".
		Door Undercut
Room CFM		Height		Width		Needed Transfer Grille Area			Jump Duct Diameter
			" by		"	40.57 sq in	or	0.28 sq ft	8.4 inches
If the free area of grille is					%
	and Side (a) is				"	Side (b) should be		10.09 inches
The calculated grille size 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 information on transfer grilles from this link;
The Building Science Corporation.