Conception Calculator

Estimated Conception Date
–
most likely date of conception

📅 Estimated Conception Window

Likely Range–

What Is a Conception Calculator?

This calculator estimates when conception likely occurred, working backward from either a known or estimated due date, or forward from the first day of your last menstrual period — both based on the standard 40-week (280-day) pregnancy timeline.

How to Use the Conception Calculator

  1. Choose whether you know your due date or the first day of your last period.
  2. Enter that date.
  3. View the estimated conception date and window.

Conception Estimation Formula

From Due Date
Conception Date ≈ Due Date − 266 days
From LMP
Conception Date ≈ LMP + 14 days

Worked Example

Example: Due Date of October 8

Conception date: October 8 − 266 days ≈ January 15

Likely window: roughly January 12–18 (accounting for typical variation in ovulation timing)

Understanding Your Results

This provides an estimate, not an exact date, since ovulation timing naturally varies somewhat even in regular cycles. The conception window accounts for this typical variation, spanning several days around the single estimated date.

Important Notes

  • This calculator assumes a fairly typical 28-day cycle; conception timing can vary more with irregular cycles.
  • An early ultrasound provides the most medically reliable dating, if precise timing matters (e.g., for legal or medical purposes).
  • Sperm can survive several days in the reproductive tract, and the egg is viable for roughly 12-24 hours, which is why conception can occur across a range of days around ovulation, not just a single day.

Frequently Asked Questions

By counting backward roughly 266 days from a due date, or forward about 14 days from the first day of the last menstrual period, assuming a typical cycle.

It's a reasonable estimate but not exact, since ovulation timing varies naturally, especially with irregular cycles — actual conception could fall within a several-day window around the estimated date.

For precise dating needs, an early ultrasound from a healthcare provider is generally considered more reliable than calculator-based estimates.

Because sperm can survive several days in the reproductive tract and the egg remains viable for about 12-24 hours after ovulation, conception can technically occur across a few different days around ovulation.

Yes — longer or shorter cycles shift the likely ovulation and conception window; this simplified calculator assumes a standard 28-day cycle.