I'm trying to calculate business days between 2 date lists in SharePoint, and I found a formula that works great here:
http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Server/MS-SharePoint/Q_22144390.html
=(DATEDIF([Start Date],[Due Date],"D"))-INT(DATEDIF([Start Date],[Due Date],"D")/7)*2-IF((WEEKDAY([Due Date])-WEEKDAY([Start Date]))
Ram.