A due date calculator and a conception date calculator are usually presented as separate tools — but they're running the exact same formula in opposite directions.
Naegele's rule calculator, the standard forward estimate
Naegele's rule adds 280 days (40 weeks) to the first day of your last period. Starting from June 1, that lands on a due date of March 8 — with an estimated conception date of June 15 (roughly 14 days after the last period, when ovulation typically occurs).
Due date calculator, running the exact same math backward
If you already know a due date (from an ultrasound, for example), subtracting 280 days gives the estimated last period, and subtracting 266 days gives the estimated conception date directly — running the March 8 due date backward gives the identical June 15 conception estimate as the forward calculation.
Conception date calculator, why one tool covers both
Since both directions use the same underlying formula, this calculator accepts either a last period date or a known due date as the starting point — rather than requiring two separate tools for what's really one calculation.
