Calculating due date before OB appt

Nadia
Does anybody know how to tell your exact due date before making your first appointment with an OB? I'm actually still searching for an OB, so it may be up to a month before that appointment happens. I've seen online due date calendars, but some of them come out different, depending on the length of your cycle, etc. My last period started on September 24, lasted 5 days, and the number of days between the start of my August period and my September period is 32. Let me know :)