MasterTool

Conception Calculator

Estimate the likely conception date and window from a due date or from the first day of your last period.

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What this tool does

Estimate the likely conception date and window from a due date or from the first day of your last period. Date arithmetic on average cycle assumptions. Actual conception timing varies with ovulation day and cycle variability; this cannot establish paternity or exact conception timing.

How to use the Conception Calculator

  1. Enter or select i know my.
  2. Enter or select date (yyyy-mm-dd).
  3. Enter or select typical cycle length (days).
  4. Read the calculated result; change any measurement to compare alternatives.

Formula

from due date: conception ≈ due date − 266 days; from LMP: conception ≈ LMP + (cycle − 14) days; window shown as ±3 days
mode
I know my
date
Date (YYYY-MM-DD)
cycle
Typical cycle length (days)

Date arithmetic on average cycle assumptions. Actual conception timing varies with ovulation day and cycle variability; this cannot establish paternity or exact conception timing.

Worked example

For conception calculator, the following measurements illustrate the exact method: I know my: duedate; Date (YYYY-MM-DD): 2027-06-15; Typical cycle length (days): 28.

Inputs

  • I know myDue date
  • Date (YYYY-MM-DD)2027-06-15
  • Typical cycle length (days)28

Result

  • Estimated conception date2026-09-22
  • Likely conception window2026-09-19 through 2026-09-25

Results explained

Estimated conception date
Estimated conception date from the formula above. Date arithmetic on average cycle assumptions. Actual conception timing varies with ovulation day and cycle variability; this cannot establish paternity or exact conception timing.
Likely conception window
Likely conception window from the formula above. Date arithmetic on average cycle assumptions. Actual conception timing varies with ovulation day and cycle variability; this cannot establish paternity or exact conception timing.

Frequently asked questions

A full-term pregnancy averages 266 days from conception to the due date, so conception is estimated as due date minus 266 days, with a ±3-day window shown for realism.

Ovulation (and conception) is estimated at cycle length minus 14 days after the LMP — day 14 of a 28-day cycle — because the luteal phase is typically about 14 days.

Within a few days for regular cycles; less so with irregular cycles, since ovulation day is what actually matters and it varies. Ultrasound dating in early pregnancy is more precise.

No. The window is several days wide and sperm survival extends it further. It must never be used for paternity questions — that requires DNA testing.

280 days is the LMP-to-due-date span (Naegele's rule) which includes the roughly 14 pre-ovulation days; 266 days is the conception-to-birth span.