Conception Date Calculator
Estimate the probable date of conception based on LMP or due date. Conception typically occurs around day 14 of a 28-day menstrual cycle, approximately 266 days before delivery.
This free online conception date calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.
Enter 0 if calculating from due date instead
Enter 0 if calculating from LMP instead
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Conception Date Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Conception Date Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
Formula Reference
Conception Date Calculator Formula
See calculator inputs for the governing equation
Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.
When to Use This Calculator
- •Use the Conception Date Calculator for personal health tracking and wellness monitoring, establishing a baseline and tracking changes over time.
- •Use it when recording fitness metrics to track progress toward health or athletic goals.
- •Use it to compare measurements before and after a lifestyle, diet, or training change to quantify the impact.
- •Use it as a conversation starter before a doctor's appointment, bringing objective data to discuss with a healthcare professional.
About This Calculator
The Conception Date Calculator is a free health and wellness calculation tool designed for personal use and general informational purposes. Estimate the probable date of conception based on LMP or due date. Conception typically occurs around day 14 of a 28-day menstrual cycle, approximately 266 days before delivery. This calculator provides reference values based on established health screening formulas and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Results may vary based on individual factors not captured by the calculation. Consult a qualified healthcare provider for personalized health guidance. All calculations are performed locally in your browser — no personal health data is transmitted or stored.
About Conception Date Calculator
The Conception Date Calculator estimates when conception (fertilization) most likely occurred based on either the last menstrual period (LMP) or the expected due date. In a standard 28-day menstrual cycle, ovulation occurs around day 14, and the fertile window spans approximately days 10-16. Since sperm can survive in the reproductive tract for up to 5 days and the egg remains viable for 12-24 hours after ovulation, the conception window is wider than a single day. This calculator provides the most probable conception date and calculates fetal age (as distinct from gestational age, which counts from LMP).
The Math Behind It
Formula Reference
Conception from LMP
Conception ≈ LMP + 14 days
Variables: Assumes 28-day cycle with ovulation on day 14
Conception from Due Date
Conception ≈ Due Date - 266 days
Variables: 266 days = 38 weeks gestational post-conception
Worked Examples
Example 1: From LMP
LMP was 60 days ago
Conception likely occurred approximately 46 days ago. Fetal age: 6 weeks.
Example 2: From Due Date
Due date is 200 days away
Conception likely occurred approximately 66 days ago. Fetal age: 9 weeks.
Common Mistakes & Tips
- !Confusing fetal age with gestational age. Fetal age starts at conception (~2 weeks after LMP). Gestational age starts at LMP.
- !Assuming conception date equals the date of intercourse. Sperm can survive up to 5 days, so conception may occur days after intercourse.
- !Using day 14 for ovulation in non-28-day cycles. In a 35-day cycle, ovulation is approximately day 21, not day 14.
Related Concepts
Used in These Calculators
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Frequently Asked Questions
Can I know the exact date of conception?
Not precisely. Even with tracked ovulation, conception can occur within a ~24-hour window of ovulation, and sperm from intercourse days earlier may be responsible. The estimate is typically accurate to within 2-3 days.
What is the difference between fetal age and gestational age?
Gestational age counts from the first day of the last menstrual period (LMP), which is approximately 2 weeks before conception. Fetal age counts from the estimated date of conception. All medical references use gestational age.
How do irregular cycles affect conception date estimates?
Significantly. In irregular cycles, ovulation timing is unpredictable, making LMP-based estimates less reliable. Ultrasound dating in the first trimester is the most accurate method for establishing conception timing in these cases.