Conception Calculator – Estimate Your Conception Date
Our free conception calculator helps you estimate the most likely date of conception based on your due date, last menstrual period (LMP), or birth date. Understanding when conception occurred can be helpful for prenatal planning, confirming pregnancy timelines, or simply satisfying curiosity. By accounting for your individual cycle length, our tool provides a personalized conception window rather than a one-size-fits-all estimate.
Choose the date you know to estimate the conception date.
Enter your due date, last menstrual period start date, or birth date in YYYY-MM-DD format.
Enter your average cycle length. Default is 28 days.
Your results will appear here
How to Use This Calculator
1. Select your calculation method: choose 'Due Date' if you know your expected delivery date, 'Last Menstrual Period' if you know when your last period started, or 'Birth Date' if your baby has already been born. 2. Enter the relevant date in YYYY-MM-DD format (for example, 2026-01-15). 3. Enter your average menstrual cycle length in days — if you're unsure, leave the default at 28 days. 4. Click Calculate to instantly see your estimated conception date, the full conception window, your estimated due date, and your current gestational age and trimester.
How the Conception Calculator Works
The conception calculator uses well-established obstetric formulas to work backwards or forwards from a known date in your pregnancy timeline. Here's the science behind it:
Ovulation and the Fertile Window
Conception occurs when a sperm fertilizes an egg during or shortly after ovulation. Ovulation typically happens approximately 14 days before the start of your next period. For a standard 28-day cycle, this means ovulation occurs around day 14. For longer or shorter cycles, ovulation shifts accordingly — hence why we ask for your cycle length.
- Ovulation day estimate: Cycle length minus 14 days from the start of the LMP
- Fertile window: Typically spans 5 days before ovulation through 1 day after
- Sperm viability: Sperm can survive up to 5 days inside the female reproductive tract
- Egg viability: An egg is viable for 12–24 hours after ovulation
Calculating from a Due Date
If you know your estimated due date (EDD), the calculator works backwards by subtracting 280 days (40 weeks) to estimate your last menstrual period, then adds the ovulation offset to estimate conception. This is Naegele's Rule in reverse.
Calculating from the Last Menstrual Period
The most common obstetric method. Your LMP date plus your cycle's ovulation offset (cycle length minus 14) gives the estimated conception date. Your due date is then calculated as LMP plus 280 days.
Naegele's Rule
Naegele's Rule is the standard formula for calculating a pregnancy due date: add one year, subtract three months, and add seven days to the first day of the last menstrual period. This approximates a 40-week (280-day) gestation from LMP.
Gestational Age vs. Fetal Age
Gestational age is measured from the LMP, meaning at the estimated conception date, you are already considered approximately 2 weeks pregnant in clinical terms. Fetal age (actual age of the embryo) is roughly 2 weeks less than gestational age.
Trimester Breakdown
- 1st Trimester: Weeks 1–12 from LMP (conception through early fetal development)
- 2nd Trimester: Weeks 13–26 (rapid growth, movements often felt)
- 3rd Trimester: Weeks 27–40 (final development, preparation for birth)