Conception Calculator

How to use this conception date calculator

What this calculator does

This calculator estimates the calendar relationship among a due date, the first day of the last menstrual period (LMP), average cycle duration, and conception date. It can work backward from an estimated due date, forward from a known or assumed conception date, or from LMP plus cycle length. It also shows the estimated gestational age in weeks and days as of today. These are planning estimates, not a way to prove the exact day fertilization occurred, establish parentage, or replace clinical pregnancy dating. Medical dating may change after ultrasound or other clinical assessment; the NCBI pregnancy-dating review explains why accurate gestational age matters and why first-trimester ultrasound is generally more precise than calendar dating alone.

When to use it

• Estimate a likely conception date after a clinician gives you an estimated due date.
• Translate a known LMP and a regular 21 – 35 day cycle into an estimated conception date and due date.
• Work forward from a known or closely tracked conception date for calendar planning.
• See the corresponding gestational age when the estimated pregnancy is currently in progress.

How to calculate

1. The calculator opens with a ready-to-use demonstration: a due date of February 15, 2027, LMP of May 11, 2026, 28-day cycle, and conception date of May 25, 2026. The example results and Excel workbook are available immediately.
2. Edit any one of the three date fields. The date you edit becomes the calculation basis, and the other dates update automatically. If you edit Average cycle duration, the calculator keeps the most recently edited date as the basis.
3. Read Estimated conception date, Estimated due date, Estimated LMP, You are..., and Average cycle duration in the results panel. The first three are linked calendar estimates; You are... is gestational age counted from LMP.
4. Select Download Excel to export the current validated inputs and results as an OOXML workbook. Select Reset to clear all three date fields and return cycle duration to 28 days. Reset also clears calculated results and disables Excel export until a complete valid date is entered again.

Input guide

Due date is an optional calculation anchor entered as a calendar date using the browser's date control. A realistic example is February 15, 2027. When you edit it, the calculator subtracts 266 days to estimate conception. A common mistake is treating the result as an exact fertilization date; due dates themselves are estimates.

1st day of last period (LMP) is another optional calculation anchor. Enter the first day bleeding began, not the day the period ended. In the demonstration it is May 11, 2026. With a 28-day cycle, conception is estimated 14 days later. The NHS due-date guidance describes the standard use of LMP and cycle length and notes that pregnancy dating may be refined at the dating scan.

Average cycle duration is a required select value from 21 to 35 days; the demonstration uses 28 days. Longer cycles move the LMP-based conception estimate later, and shorter cycles move it earlier. The estimate assumes ovulation occurs about 14 days before the next period, which is only an approximation. The NHS menstrual-cycle overview explains that ovulation timing varies and is difficult to pinpoint exactly.

Conception date is an optional calculation anchor for situations where ovulation or conception timing is known more closely. The demonstration uses May 25, 2026. Editing it adds 266 days for the estimated due date and uses cycle length to infer an LMP date. Do not enter the date of a positive pregnancy test as though it were the conception date; those events occur at different times.

Output guide and worked example

Estimated conception date is the central result and is always a calendar estimate. Estimated due date is 266 days after the estimated conception date. Estimated LMP is the corresponding first day of the last period under the selected cycle assumption. You are... shows gestational age from LMP in completed weeks and remaining days, matching the medical convention that gestational age is counted from the first day of the last menstrual period; MedlinePlus explains gestational-age dating. Average cycle duration repeats the cycle assumption used in the calculation.

For the demonstration, February 15, 2027 minus 266 days gives May 25, 2026 as the estimated conception date. With a 28-day cycle, the ovulation offset is 28 – 14 = 14 days, so the corresponding LMP is May 25 minus 14 days = May 11, 2026. On August 9, 2026, the interval from that LMP is 90 days, or 12 completed weeks and 6 days. Those same typed dates and numeric values are used in the on-screen results and the downloadable workbook.