Conception date calculator
Work backwards from your due date to estimate when you conceived. This is a guide only — your care team confirms dating.
Don't have a due date yet? Use the due date calculator
What a conception date means
Conception is the day sperm fertilises an egg. That moment is rarely known exactly: sperm can live for up to about five days, and an egg for about a day. This tool estimates the most likely day from your due date, assuming a 28-day cycle and ovulation on day 14.
How this estimate is calculated
We subtract 266 days (38 weeks) from your estimated due date. That matches the usual gap between conception and a 40-week due date counted from the last period. If you conceived through IVF, use the transfer date your clinic recorded.
Can the date be wrong?
Yes. Cycle length, late ovulation, and dating scans all move the estimate. Treat the result as a guide, not a record of the exact day you conceived.
This calculator is an educational estimate, not medical advice. It does not replace ultrasound dating or clinical care.