Calculate Duration (days) between two dates but only calculate weekdays.

I have two dates fields side by side in a form.  Site Plan Requested and Site Plan Returned.  I have a field beside these called "Days to Complete" that calculates the duration.  Right now the field is using a simple operator (-) to handle the duration calc, but it includes weekends.  So when I add a date in both fields with the weekdaysub formula, the Days to Complete field is blank.