Conception Date Calculator

One worked example

Say your last period started on March 1. Add roughly 14 days for typical ovulation timing and you land near March 15 as the estimated conception day, with the practical window spanning March 12 through March 18. Run the same date the other direction, as a due date of December 5, and subtracting 266 days lands close to the same point in mid March, since both formulas describe the same 280 day pregnancy from two different anchor points.

Why the conception date is always a range

Two things push conception away from being a single, knowable day. First, ovulation itself drifts a little from cycle to cycle even in people with generally regular periods. Second, sperm can remain capable of fertilizing an egg for several days before ovulation happens, so intercourse well before the egg is released can still result in conception. Together, these push any date-based estimate toward a window rather than a point. This calculator keeps that window narrow and practical, about 3 days either side of the central estimate, rather than trying to capture every possible edge case.

When to reach for a wider estimate instead

This page is built for speed: one date in, one answer out, with a tight window attached. If your cycle runs noticeably longer or shorter than 28 days, or you want to see both calculation methods laid out side by side with the underlying formulas explained, the conception calculator elsewhere on this site walks through that comparison in more depth. Either page draws on the same 266 and 14 day figures; this one simply gets you a number faster when you already know which date you are starting from.