No calculator can name the exact day. What it can honestly give you is a realistic window, and the single most likely date inside it.
Free pregnancy tool
Pregnancy Conception Calculator
Work backward to an estimated conception window, from either your last period or a known due date.
Runs entirely in your browser. Nothing you enter is sent anywhere.
Why this is a window, not a single confirmed date
Conception can only happen within a narrow fertile window around ovulation, sperm can survive inside the body for up to about 5 days waiting for an egg, while the egg itself is typically viable for around 24 hours after release. From a last-period date, ovulation is estimated at cycle length minus 14 days, then the calculator shows the earliest realistic date (accounting for sperm survival), the single most likely date (ovulation itself), and the latest realistic date. Working backward from a due date is less precise, since a due date alone carries no information about your actual cycle length, it assumes a standard 266-day gestation from conception with no adjustment. Neither method can give a single confirmed date, only a fertile-window estimate.
Conception calculator FAQ
Can this tell me the exact day I conceived?
No, and no calculator honestly can. Conception can only be estimated within a fertile window, since sperm can survive inside the body for days waiting for ovulation. This tool gives you the earliest realistic date, the most likely date, and the latest realistic date, not a single confirmed answer.
Why does cycle length matter for estimating conception?
Ovulation, and with it the real conception window, happens roughly 14 days before your next period, not 14 days after your last one, on a standard 28-day cycle those are the same, but on a longer or shorter cycle they diverge, which is why an accurate cycle length matters.
How accurate is the due-date-based estimate?
Less precise than the last-period method, because a due date alone carries no information about your actual cycle length. It simply subtracts the standard 266-day gestation period from conception to due date, giving a single estimate rather than a window.
Can I use this for legal or paternity purposes?
No. This is a statistical estimate based on average cycle timing, not a medical or legal determination. Paternity and legal conception questions need clinical dating (such as an early ultrasound) or genetic testing, not an online calculator.
Is my data private?
Yes. Every calculation runs in your browser. Nothing you enter is sent to a server, logged, or stored.
Free conception calculator by ANUPRESS
Why a window, and not one confirmed date
Sperm can survive inside the body for up to about five days waiting for ovulation, while a released egg is typically viable for only around 24 hours. That biological reality is why this calculator shows an earliest realistic date, a most-likely date at estimated ovulation, and a latest realistic date, rather than pretending to know a single exact day nobody can actually confirm without a clinical ovulation test.
Two starting points, two different confidence levels
Estimating from your last period and real cycle length gives you an actual window, since ovulation timing depends directly on cycle length. Working backward from a due date is necessarily less precise: a due date alone carries no cycle-length information, so that path gives a single estimated point rather than a genuine range.
What this estimate isn’t for
This is a statistical estimate built on average cycle timing, not a medical or legal determination of anything. Questions that actually require certainty, paternity, legal conception dates, or anything with real consequences riding on the answer, need clinical dating (an early ultrasound, for instance) or genetic testing, not an online calculator. For the pregnancy itself, our Due Date Calculator and Pregnancy Calculator cover the due date and week-by-week tracking this tool doesn’t.