What is Conception Date?
The Conception Date Calculator estimates when conception likely occurred based on either your due date or your last menstrual period (LMP). This is one of the most searched pregnancy calculators after the due date calculator.
Conception typically occurs during ovulation, approximately 14 days after the first day of your last menstrual period (for a regular 28-day cycle). However, sperm can survive up to 5 days in the reproductive tract, so the fertile window spans about 6 days — from 5 days before ovulation to 1 day after.
If you know your due date: subtract 266 days (38 weeks) to estimate the conception date. If you know your LMP: add 14 days to estimate ovulation/conception. These are estimates — actual conception may vary by a few days depending on your cycle length and ovulation timing.
Pregnancy is counted from the LMP (not conception), which is why it's said to last 40 weeks even though actual pregnancy from conception is about 38 weeks.
Also check our Pregnancy Due Date Calculator and Age Calculator.
Formula
From Due Date: Conception Date = Due Date − 266 days (38 weeks)
From Last Menstrual Period (LMP): Conception Date = LMP + 14 days (assumes 28-day cycle)
Fertile Window = Conception Date − 5 days to + 1 day
Due Date from LMP = LMP + 280 days (40 weeks)
Example — Due Date June 15, 2026: Conception ≈ September 22, 2025 Fertile window: Sep 17-23, 2025
Example — LMP September 8, 2025: Conception ≈ September 22, 2025 Due date ≈ June 15, 2026
How to use this Conception Date Calculator?
1. Select whether you're calculating from your Due Date or Last Period (LMP). 2. Enter the date (year, month, day). 3. See estimated conception date, fertile window, and current weeks pregnant.
Note: This assumes a regular 28-day menstrual cycle. If your cycle is longer (e.g., 35 days), ovulation occurs later, and conception date shifts accordingly. Ultrasound dating is more accurate than LMP-based calculations.