How to calculate your conception date
This guide shows how to calculate your conception date by hand, with examples you can check yourself.
From your due date
Conception date = due date − 266 days. Example: a due date of 14 May 2027 gives a likely conception date of 21 August 2026.
The window when sex most likely led to the pregnancy is the 6 days ending on that day: 16 to 21 August 2026.
From your last period
In a 28-day cycle, ovulation is about 14 days after the first day of your period. In a longer or shorter cycle, ovulation is about 14 days before your next period would start.
Example: a 32-day cycle that started on 1 October gives ovulation around 19 October.
Why it is only an estimate
- Ovulation can come early or late, even in regular cycles.
- Sperm can live up to 7 days after sex.
- An early ultrasound is the most accurate way to date a pregnancy.
Use the conception date calculator to do the math for you.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date · checked October 24, 2026
- NHS: Periods and fertility in the menstrual cycle · checked October 26, 2026