How the conception date is estimated
Conception usually happens around ovulation, roughly two weeks after the first day of your last period. This calculator works backward from your due date, or forward from your last period, to estimate when conception likely occurred. Because it is based on averages, it gives a likely window rather than an exact day.
Worked example
For a due date of 8 October 2026, the estimated conception date is around 15 January 2026, roughly 266 days earlier.
Good to know
Actual conception depends on your individual cycle and ovulation timing, which vary. Treat the result as an estimate for interest or planning, not a medical or legal determination.
Frequently asked questions
How is the conception date worked out?
It is estimated as about two weeks after the first day of your last period, or roughly 266 days before the due date. It gives a likely window, not an exact day.
Is the conception date exact?
No. It is based on average cycle timing. Real ovulation and conception vary from person to person, so treat it as an estimate.
Can this tell me the exact day?
No. It gives an estimated window. Actual conception depends on ovulation timing and can vary by several days.
What if I know my last period instead?
Use the due date calculator to go from your last period to a due date, then this tool for conception.
Is it reliable for paternity?
No. These estimates are not precise enough for legal or paternity purposes.