Basically you have to know your ov date to caluculate it. If you ov cd14 and your af comes on cd 28- you have a lp of 14 days

....count forwards from ovulation counting the day after your ov as day 1.
By the way, i have a 30 days cycle..ov cd17 so have lp of 13 days.
LP can be anywhere between 11-16 days. Any shorter and the endimitrium doesnt have time to build up (your period and food for bubba) and the embryo doent have anything to implant into- other problems with luteal phase defect, LPD (if too short or too long) is poor follicle stimulation and short lived egg.
Hope this helps
