Pregnancy Conception Calculator
Estimate the conception date and fertile window from a due date or last menstrual period.
Runs 100% in your browser — nothing is uploaded.
Result
Formula
Conception ≈ EDD − 266 days adjusted for cycle length.
About this calculator
Estimate the most likely conception date and fertile window based on a known due date and typical cycle length. The calculator uses a reverse Naegele's rule approach.
How to use
- 1
Enter your baby's estimated due date.
- 2
Enter your typical menstrual cycle length in days.
- 3
Click Calculate to see the estimated conception date and fertile window.
Frequently asked questions
How accurate is the conception date estimate?
The estimate is based on the assumption that conception occurs 266 days before the due date, which is accurate for a typical 28-day cycle. However, due dates themselves are estimates, and only about 5% of babies are born exactly on the due date. The actual conception date can vary by a few days.
What if my cycle length is not 28 days?
The calculator adjusts for cycle length by shifting the conception date by the difference from 28 days. For example, a 35-day cycle shifts conception 7 days later. This assumes ovulation occurs 14 days before the next period, which is typical.
Can I use this calculator if I know my last menstrual period instead of a due date?
This specific calculator requires a due date. If you know your LMP, you can use the Due Date Calculator to estimate your due date first, then use that result here.
Please note: Results are estimates for general information only and are not professional or medical/financial advice.