I'm playing with one of those due date calculators and it's asking me the length of my cycle. FF says my least count was 25 days and most was 27 with an obvious average of 26 days.
How do I know what it is for this past cycle? Is it from my LMP to the expected AF date? BFP date?
I believe most EDD calculators use a 14 day LP as a standard. So, it will count back 14 days from the last day of your cycle length and that will be your estimated O date and what your EDD is based on. If you know your O date some calculators will let you use that to calculate your EDD.
I never knew my cycle length either because my O day usually varies. When I was charting while TTC #2 I actually had 5 different O days over 5 cycles. But my luteal phase (LP) was always 12 days so once I O'd I knew AF was due 13 days later. She was on time, every time, when I wasn't preggers. So for me if I used a standard EDD calculator with my expected cycle length it would result with the expected date of conception 2 days off because my LP is 2 days shorter than the standard.
This website has a good discussion of all this here...I like their calculator too because you can enter either LMP or Ovulation/conception. Here's the calculator. I like to get an accurate EDD b/c I'm just a bit anal but it's good to keep in mind, especially at about 40 weeks preggers, that it is just an estimate.