💫 Conception Date Calculator
Pregnancy lasts about 266 days from conception to birth, so a known due date lets you walk the calendar backwards to the likely conception date. Early ultrasound dating makes this estimate noticeably sharper.
Because sperm can survive several days, conception may fall anywhere in roughly a one-week window — the calculator shows that window alongside the single best-estimate date.
How to use this calculator
Pick a known due date and the tool counts back to estimate the conception date.
- Pick the Due date on the date field.
- Read the headline estimated conception date.
- Check the likely window around it.
- Note the estimated last-period date underneath.
Frequently asked questions
How is the date worked out?
Pregnancy averages 266 days from conception to birth, so the tool subtracts 266 days from the due date. A due date of 25 December 2026 points to conception around 3 April 2026.
Why show a window instead of one day?
Sperm can survive several days, so conception may fall anywhere in roughly a one-week span. The calculator shows three days either side of the best estimate.
How accurate is the result?
It is only as good as the due date: one based on an early ultrasound is sharper than one from period dates alone. Treat it as an estimate, not a precise timestamp.
What is the LMP row for?
LMP is the first day of the last menstrual period, the traditional dating start, estimated here as 280 days before the due date. Doctors use it because it is easy to anchor.