how far along am i??

so I have an appointment for my obgyn on Thursday and I'm gonna tell her I think I got a faint positive on a test because that's what I think happened lol. so if im pregnant how can I tell how many weeks I am? because the first response website said (from lmp) my due date is October 6 and I'm 3weeks +6 days but on my nurture app i put in October 6 as my due date and it says 3 weeks and 1 day? I'm very confused I know my doctor will tell me but I'm impatient lol