Conception Calculator

Estimate your likely conception date from the first day of your last period or from your due date, plus your estimated due date, last period date, and fertile window.

Quick Facts

Ovulation timing
Ovulation ≈ cycle length − 14 days
Ovulation usually occurs about 14 days before the next period starts, regardless of overall cycle length.
Naegele's rule
Due date ≈ LMP + 280 days
Standard estimate of a 40-week pregnancy for a typical 28-day cycle.
Gestation length
≈ 266 days from conception
Full-term pregnancy is about 38 weeks from the actual conception date.
Fertile window
≈ 6 days
Sperm can survive up to 5 days; the egg is viable about 12-24 hours after ovulation.

Your Results

Calculated
Estimated Conception Date
-
Most likely date of conception (ovulation day)
Estimated Due Date
-
Naegele's rule, ≈40 weeks from LMP
First Day of Last Period
-
Estimated or entered LMP date
Fertile Window
-
Days with the highest chance of conceiving

Ready

Choose a calculation method, enter a date, and press Calculate.

How the Conception Date Calculator Works

This calculator estimates the date you most likely conceived, using either the first day of your last menstrual period (LMP) or your due date (EDD) as a starting point. Both paths rely on the same clinical convention: ovulation — and therefore conception — typically occurs about 14 days before the start of the next period, and a full-term pregnancy lasts about 266 days from conception (roughly 280 days, or 40 weeks, from the LMP under Naegele's rule for a standard 28-day cycle).

How the calculation works

If you calculate from your last period, the tool adds (cycle length − 14) days to your LMP date to estimate ovulation/conception, and adds (cycle length + 252) days to estimate the due date — this reduces to the familiar "LMP + 280 days" only when your cycle is exactly 28 days. If you calculate from your due date, the tool works backward: conception date = due date − 266 days, and the implied LMP date = conception date − (cycle length − 14) days. The fertile window is shown as the five days before the estimated ovulation day through the ovulation day itself, since sperm can survive in the reproductive tract for up to five days while the egg remains viable for roughly 12–24 hours after release.

Common mistakes

  • Assuming a 28-day cycle when yours is different: ovulation timing shifts with cycle length — a 35-day cycle ovulates later (around day 21) than a 24-day cycle (around day 10), which meaningfully changes the estimated conception date.
  • Treating the estimate as a confirmed date: this tool gives the statistically most likely day based on average timing; actual ovulation can vary cycle to cycle even for the same person.
  • Confusing LMP with conception date: the last menstrual period starts about two weeks before ovulation, not on the day of conception itself.

When timing matters

Estimated conception dates are commonly used to cross-check due dates, to satisfy paperwork that asks for an approximate conception window, or simply out of curiosity about when a pregnancy began. For clinical dating of a pregnancy, an early first-trimester ultrasound is more accurate than calendar-based estimates, especially for people with irregular cycles. This calculator is an educational estimate, not a medical or legal determination — discuss dating and due-date questions with your healthcare provider.

Frequently Asked Questions

How is conception date calculated from my last period?
Ovulation typically occurs 14 days before your next expected period, regardless of cycle length. So the estimated conception date equals the first day of your last menstrual period (LMP) plus (cycle length − 14) days.
How is conception date calculated from a due date?
A full-term pregnancy lasts about 266 days from conception. So the estimated conception date equals your due date minus 266 days. Naegele's rule separately estimates the due date as the last period date plus 280 days (40 weeks) for a standard 28-day cycle.
What is the fertile window?
Sperm can survive up to about five days in the reproductive tract, while an egg is viable for roughly 12–24 hours after ovulation. That creates a fertile window of about six days: the five days before ovulation plus the day of ovulation itself, which is also the most likely conception date.
How accurate is a conception date estimate?
This calculator assumes a regular cycle and average ovulation timing, so the result is an estimate, not a diagnosis. Irregular cycles, late or early ovulation, and other individual factors can shift the actual date. An early ultrasound dating scan is the most accurate way to confirm gestational age; talk with your healthcare provider for a clinical estimate.

Related calculators