Conception Date Calculator
Estimate when conception probably happened, and the window of days around it.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
from a due date: due date − 266 days from the last period: first day + cycle length − 14 days window: the 5 days before, and the day itself
Conception is taken as the day of ovulation, about 14 days before the next period. Sperm can survive for several days, so sex in the days before can lead to conception.
What to watch out for
Use a due date from a dating scan if you have one. Ovulation timing varies from cycle to cycle, so treat the window as a guide.
Frequently Asked Questions
How do I work out conception from a due date?
Take away 266 days. A due date of 8 October 2027 gives about 15 January 2027, with a window from 10 January 2027.
How do I work it out from the last period?
Add the cycle length and take away 14 days. From 1 January 2027 with a 28-day cycle, that is about 15 January 2027.
Can this show the exact day?
No. Ovulation timing varies, sperm can survive for several days, and a due date is itself an estimate. Irregular cycles make it less reliable still.