Conception date calculator | Determine Your Fertility Window
Use our Conception Date Calculator to find your likely conception date by entering your due date or last menstrual period. It estimates your conception window to clarify your pregnancy timeline.
What is a conception date calculator?
Estimate your likely conception date, fertile window, due date and current gestational age. Calculate from your last menstrual period (LMP) or work backwards from your due date.
How the conception date is estimated
From last period: conception ≈ first day of last period + (cycle length − 14) days
From due date: conception ≈ due date − 266 days
Due date (Naegele's rule): last period + 280 days
Example: if your last period started on the 1st with a 28-day cycle, conception was likely around the 15th and the due date is about 40 weeks later. The exact day is rarely known.
Pregnancy dating at a glance
| Milestone | Timing |
|---|---|
| Last menstrual period (LMP) | Day 0 |
| Ovulation / likely conception | ~14 days after LMP |
| Fertile window | ~5 days before to day of ovulation |
| Pregnancy length (from LMP) | ~280 days (40 weeks) |
| Due date from conception | ~266 days (38 weeks) |
How to use
- Choose whether to calculate from your last period or your due date.
- Enter the date and, for LMP, your average cycle length.
- Press Calculate to see your conception date and fertile window.
- Review your due date, gestational age and pregnancy milestones.
What is a conception date calculator?
A conception date calculator estimates when you likely conceived. Enter the first day of your last period or your due date, and it returns your probable conception date, fertile window, estimated due date and current gestational age. Conception usually happens around ovulation, roughly 14 days after a 28-day cycle begins.
