Conception Date Calculator
Calculate the likely date of conception based on the due date or the date of the last period.
Tip: Conception usually happens 11–21 days after the first day of the last period — not on the day of intercourse, since sperm live for up to 5 days.
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About this digital tool
The conception date calculator estimates the likely day conception happened. You can start from the date of your last period or from the predicted due date. The result is a window of several days, not a single day, together with the estimated ovulation date and the current week of pregnancy.
If you start from the due date, the calculator subtracts 266 days (38 weeks from conception); if you start from the last period, it assumes ovulation on day 14 of the cycle. Exact dates are impossible to determine, because sperm live for up to several days and ovulation varies from cycle to cycle. Use it as an informational estimate; for accurate dating of the pregnancy see your gynaecologist.
How to use
- Choose the method: from the due date or from the date of the last period
- Enter the relevant date
- Or enter the date of your last period
- See the likely conception window
- See the current week of pregnancy
Frequently asked questions
Can I know the exact date of conception?
Not with 100% accuracy. The calculator gives a 5-day window with a high probability. Only if you've had IVF or tracked ovulation by ultrasound can you know it exactly.
How is it calculated from the due date?
Due date minus 266 days (38 weeks) = approximate conception date. Or: due date minus 280 days = first day of the period, plus 14 days = ovulation/conception.
Why does it differ from the day of intercourse?
Sperm live 3–5 days in the female reproductive tract, while the egg lives only 12–24 hours. So intercourse 5 days before ovulation can lead to conception on the day of ovulation.
Why is the result a window and not an exact date?
Because ovulation and sperm lifespan vary. Conception can happen several days after intercourse, so the window gives a more realistic interval.
How is conception calculated from the due date?
266 days are subtracted from the predicted due date, which is the average length from conception to birth. A window is then formed around this date.