Conception date calculator
Work backwards from a due date to the estimated conception date and six-day window, or forwards from your last period. Nothing is transmitted.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
Nothing you type here is sent anywhere.
The dates are used by JavaScript running inside this page and are discarded when you close the tab. There are no cookies, no analytics and no account. Storage on this device is off unless you switch it on below, and you can delete it in one click.
First day of one period to the first day of the next.
Ovulation to the next period. Far steadier than cycle length — 14 days if you don't know.
Two ways round the same arithmetic
Pregnancy is dated from the first day of the last menstrual period, not from conception. That convention exists because the LMP is a date people can usually remember and ovulation is not, and it means the "weeks pregnant" figure includes roughly two weeks before conception happened.
Everything follows from two numbers:
- 280 days from the first day of the last period to the due date (Naegele's rule, 40 weeks).
- 266 days from ovulation to the due date — 280 minus the usual 14-day gap.
So working backwards from a due date, conception was about 266 days earlier. Working forwards from a period, conception was about 14 days later — adjusted for your actual cycle and luteal phase, which this tool lets you set.
Conception is a window, not a day
Sperm survive in fertile cervical mucus for up to five days. Fertilisation therefore happens at ovulation, but the intercourse that caused it may have been anywhere in the preceding five days. That is why the result here is a six-day span rather than a date, and it is the single most important caveat on the page.
Ovulation timing also varies. Even in regular cycles it moves by several days between months, and any illness, travel or stress in the first half of the cycle can delay it. A conception date derived from a due date carries all of that uncertainty forward.
Which due date to start from
| Source of due date | Typical accuracy |
|---|---|
| First-trimester dating ultrasound (8–13 weeks) | Around ±5 days — the clinical standard |
| Second-trimester ultrasound | Around ±7–10 days |
| Third-trimester ultrasound | ±2–3 weeks; poor for dating |
| Last menstrual period, regular cycles | Good, if the LMP date is reliable |
| Last menstrual period, irregular cycles | Poor; a scan will usually override it |
| IVF transfer | Essentially exact — the date is known |
If you have a scan-based due date, use it. It is derived from the size of the fetus rather than from a remembered date, and it is what a maternity record will be dated on.
What this cannot tell you
It cannot establish paternity, and it should never be used to try. The window is genuinely six days wide before you account for the uncertainty in the due date itself, which adds five days or more in either direction even from a good early scan. The only reliable answer to a paternity question is a DNA test.
It also cannot tell you the sex, the exact gestational age, or whether a pregnancy is progressing normally. It is date arithmetic with the working shown, and that is all.
Only 4% of babies arrive on the due date
A due date is the midpoint of a distribution, not an appointment. Roughly 4% of births happen on the estimated date; about half occur within a week of it, and the great majority between 37 and 42 weeks. Some maternity services have moved to quoting a due week for exactly this reason.
Nothing entered here is transmitted
Dates relating to a pregnancy are sensitive, and this page is built so they cannot go anywhere: the calculation is JavaScript running in your browser, there is no backend, there are no cookies and no analytics, and local saving is off unless you turn it on. Open your network tab while you use it — you will see no requests at all.
Questions
How is the conception date calculated from a due date?
By subtracting 266 days. Pregnancy is dated from the first day of the last period, 280 days before the due date, but conception happens at ovulation, roughly 14 days later — so 280 minus 14 is 266.
Is the conception date the same as the day of intercourse?
Not necessarily. Sperm survive up to five days, so conception can occur days after intercourse. That is why this tool reports a six-day window rather than a single date, and why a conception date should never be treated as proof of anything.
How accurate is a due date from a dating scan?
A first-trimester ultrasound is accurate to about ±5 days and is the standard used to set or revise a due date. Working back from a scan-based due date is therefore more reliable than working forward from a remembered period.
Can this establish paternity or an exact date?
No. The window is genuinely six days wide, ovulation timing varies, and due dates are themselves estimates. Nothing on this page is evidence of anything; it is date arithmetic.
Is any of this sent anywhere?
No. The dates are processed in your browser and discarded when you close the tab unless you explicitly turn on local saving, which you can delete with one click.