How to use this calculator
- Enter or choose i know my.
- The estimated conception updates as you type. Use Copy result or Share to keep it.
Method
From a due date: conception ≈ due date − 266 days. From a last period: conception ≈ LMP + (cycle length − 14); due date = conception + 266 days. Window = conception − 5 days to + 1 day.
- LMP
- first day of the last menstrual period
- 266 days
- average time from conception to birth
- 280 days
- average time from LMP to birth (Naegele’s rule, 28-day cycle)
Worked example
- Conception: 12 Oct 2026 − 266 days = 19 Jan 2026.
- Window: 5 days before (14 Jan) to 1 day after (20 Jan), because sperm can survive several days.
- Equivalent LMP: 12 Oct 2026 − 280 days = 5 Jan 2026.
A due date of 12 October 2026 points to conception around 19 January 2026, from intercourse most likely between 14 and 20 January.
What the result means
The estimate is the most likely date, not a certainty. Since sperm can survive up to about a week, intercourse several days before ovulation can lead to pregnancy, which is why a window is shown.
Assumptions and limits
- Ovulation (and conception) occurs 14 days before the next expected period.
- Pregnancy lasts an average of 266 days from conception (280 days from the last period).
- Even with an ultrasound-based due date the conception estimate is uncertain by several days; with an LMP-based date it can be off by a week or more.
- Cannot establish paternity: conception dates are too imprecise for that.
- IVF pregnancies have known transfer dates; use those instead.
Questions
Can this tell me the exact day I conceived?
No. Ovulation timing varies and sperm survive several days, so dates are estimates with a window of about a week.
Why is my due date 280 days after my period, not 266?
Pregnancy is conventionally dated from the first day of the last period, about two weeks before conception in a 28-day cycle, so the two counts differ by 14 days.
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.