Conception Calculator

“When did I conceive?”

Conception happens when sperm fertilizes an egg, and that can only occur around ovulation — the day your ovary releases an egg. In a textbook 28-day cycle, ovulation lands about 14 days after the first day of your last period (LMP). So the simplest estimate of your conception date is your LMP plus roughly two weeks.

If you already know your due date, you can work backwards instead. Pregnancy lasts about 266 days from conception (which is the same as 280 days from your last period), so your conception date is approximately 266 days before your due date. Either route lands you in the same place: the few days around the time you ovulated.

How the conception calculator works

This calculator works two ways, so you can use whichever date you actually know:

• From your last period: enter the first day of your most recent period, and we add about 14 days to estimate the day you likely ovulated and conceived.
• From your due date: enter your estimated due date, and we count back 266 days to the most likely conception day.

Either way, you get a most-likely conception date plus a conception window — a short range of days around ovulation when conception could realistically have occurred. The window matters because sperm can survive for several days, so the egg can be fertilized on a different day than the one you might expect.

Why it’s a window, not a single day

A single “conception date” sounds precise, but biology is fuzzier than that. Two things widen it into a window:

• Sperm survival. Healthy sperm can live in the reproductive tract for up to about 5 days. Sex several days before ovulation can still result in conception once the egg is released.
• Ovulation timing. Not everyone ovulates on day 14. Cycle length, stress, and natural variation can shift ovulation earlier or later, which moves the conception day with it.

Because of those two factors, your true conception day sits somewhere inside a span of a few days — which is exactly what the window represents.

Conception date vs due date

The conception date and the due date are two ends of the same timeline. Your due date is set about 280 days after your last period (or 266 days after conception), while conception sits roughly two weeks after that last period. If you came here from a due date and want to flip the calculation the other way — from a period or conception date to a delivery date — use our Pregnancy Due Date Calculator. The two tools are designed to agree with each other.

A note on accuracy

Everything here is an estimate built on average cycle timing. The most accurate way to estimate when a pregnancy began is an early ultrasound (before 14 weeks), which measures the baby’s actual size rather than relying on calendar math. If your provider has given you a dating ultrasound, trust that over any calculator.

One sensitive but important point: a calculated conception date is a rough window, not a fixed fact. Because ovulation timing varies and sperm survive for days, these estimates can be off by several days in either direction. For that reason, they should never be used to settle questions of paternity — only a medical or laboratory test can speak to that. Please treat the result as a friendly planning guide, nothing more.