Counting my days wrong ?

I need a little advice I think I maybe doing this wrong and I'm trying to get pregnant this time and I don't want to mess this up. When counting your days between your menstrual cycle to get the right date for ovulation do you count the days that you were on your period ? If so I'm 28 days and if I don't count them I'm 24 days. I just want to make sure I'm doing this right so I can  try to conceive. I don't want another let down.