Conception Calculator
Work backward from your due date or last menstrual period to estimate when you conceived and when you ovulated.
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About the Conception Calculator
The conception calculator estimates the date you conceived. Pregnancy is dated from the first day of your last menstrual period, and conception usually happens about two weeks later, around ovulation. Enter your due date or your last period and your cycle length to see the likely conception window.
How to use it
- Choose whether you know your due date or your last period.
- Enter the date and your usual cycle length.
- Press Estimate conception to see the likely conception date and window.
How conception is estimated
A due date is 280 days (40 weeks) after the first day of the last period. Conception is about 266 days before the due date for a 28-day cycle. Sperm can survive up to five days, so the true date can differ by several days.
Conception timeline
| Event | When |
|---|---|
| First day of last period | Week 0 |
| Ovulation | About day 14 of a 28-day cycle |
| Conception | Within about 24 hours of ovulation |
| Missed period | About week 4 |
| Due date | Week 40 |
Frequently asked questions
How do I calculate my conception date?
Subtract 266 days from your due date, or add about 14 days to your last period for a 28-day cycle.
How accurate is a conception calculator?
It gives an estimate within a few days. Ultrasound dating is more precise.
Why does pregnancy start before conception?
Doctors count from the last period because it is easier to know than the conception date, so pregnancy includes about two weeks before conception.
Can I find the exact conception date?
Rarely. Only with tracked ovulation or assisted reproduction can you be more precise.
What if my cycle is not 28 days?
Enter your actual cycle length. Ovulation happens roughly 14 days before the next period.
Last reviewed: November 2026. Educational estimate only. Not medical advice.