Conception Calculator
Estimate your most likely conception date based on your last period and cycle length.
About the Conception Calculator
This estimates your most likely conception date forward from your last period and average cycle length, rather than backward from a due date — the two approaches land on the same underlying math but start from different known information, so use whichever date you actually have on hand.
The estimate assumes conception happens on the day of ovulation, which itself is only approximated from a typical 14-day luteal phase. Real cycles shift month to month, so treat the date shown as a likely window rather than a single confirmed day.
It's most useful for people trying to conceive who want to connect a known last-period date to an estimated conception date and a rough due date in one step, without doing the ovulation math separately first.
Everything here is calculated locally in your browser. Conception timing isn't the kind of thing most people want sitting in some company's database, so nothing you type is stored or sent anywhere.
How it’s calculated
Conception is estimated at (cycle length − 14) days after your last period started — the same ovulation-timing logic used across cycle-based fertility calculators, since conception can only happen around ovulation.
The due date is then projected 266 days past that estimated conception date, which is the standard pregnancy-length convention used clinically (equivalent to 280 days from the last period for a 28-day cycle).
Frequently asked questions
How accurate is a conception date estimated from cycle length?
It's a reasonable estimate for people with fairly regular cycles, but it depends on assuming a consistent ~14-day luteal phase — actual ovulation timing varies enough between people and even cycle to cycle that this should be treated as an approximate window.
What's the difference between this and the pregnancy conception calculator?
This one works forward from your last period and cycle length; the pregnancy conception calculator works backward from a known or expected due date. Use whichever starting date you actually have.
Why does a longer cycle push the conception date later?
Because ovulation happens roughly 14 days before your next period regardless of cycle length, a longer cycle means more days pass between the start of your last period and ovulation, shifting the estimated conception date correspondingly later.
Can I use this to confirm I'm pregnant?
No — this only estimates dates based on cycle math and doesn't detect pregnancy itself. A pregnancy test or a healthcare provider is what actually confirms whether conception occurred.
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