Conception Date Calculator

What is a conception date calculator?

A conception date calculator works backward from your due date to estimate when you most likely conceived — about 266 days (38 weeks) before the due date — and shows a likely window of roughly 5 days on either side. Enter your due date in the tool above and it instantly returns the estimated conception date, that 10-day window, and your estimated last menstrual period (LMP) date.

It answers the common reverse-lookup question — “when did I conceive?” — using the same dating math that clinics and standard due date tools rely on. Because pregnancy is normally measured from the LMP rather than from conception itself, the calculator separates those two dates clearly so you can see both.

How does the conception date calculator work?

Pregnancy is dated as 280 days (40 weeks) from the first day of the last menstrual period (LMP). Ovulation and conception happen on average about 14 days after the LMP, so conception falls about 266 days (38 weeks) before the due date. The tool subtracts those fixed offsets from your due date.

The exact method the widget uses is:

conception     ≈ due date − 266 days   (38 weeks)
likely window  ≈ conception ± 5 days
LMP            ≈ due date − 280 days   (40 weeks)

A few definitions and units:

• Due date — the estimated date of delivery (EDD) you enter, given as a calendar date.
• Conception — the day the egg was likely fertilised, estimated at about 14 days after the LMP.
• Likely window — the conception date plus or minus 5 days, because ovulation timing varies.
• LMP — the first day of your last menstrual period, about 280 days before the due date.
• Week 38 / week 40 — conception sits around 38 weeks before the due date; LMP sits at 40 weeks before.

So the difference between the LMP date and the conception date is the same 14-day gap that makes pregnancy “weeks” start about two weeks before you actually conceived.

Examples

Each example below matches the tool’s output exactly. Enter the same due date in the widget to reproduce these dates.

Example 1 — due date 8 November 2026

• Conception: 8 Nov 2026 − 266 days = Sun, 15 Feb 2026
• Likely window: about 10-20 Feb 2026 (15 Feb ± 5 days)
• LMP: 8 Nov 2026 − 280 days = Sun, 1 Feb 2026

Example 2 — due date 25 December 2026

• Conception: 25 Dec 2026 − 266 days = Fri, 3 Apr 2026
• Likely window: about 29 Mar - 8 Apr 2026 (3 Apr ± 5 days)
• LMP: 25 Dec 2026 − 280 days = Fri, 20 Mar 2026

Example 3 — due date 1 January 2027

• Conception: 1 Jan 2027 − 266 days = Fri, 10 Apr 2026
• Likely window: about 5-15 Apr 2026 (10 Apr ± 5 days)
• LMP: 1 Jan 2027 − 280 days = Fri, 27 Mar 2026

In every case the conception date is exactly 14 days after the LMP date, which is why the two dates always sit two weeks apart.

Conception date by due date

This reference table applies the same formula to several due dates so you can see the pattern at a glance. The conception date is always 266 days before the due date, and the LMP is always 280 days before.

Due date	Estimated conception	Likely window	Estimated LMP
Wed, 30 Sep 2026	Wed, 7 Jan 2026	2-12 Jan 2026	Wed, 24 Dec 2025
Sun, 8 Nov 2026	Sun, 15 Feb 2026	10-20 Feb 2026	Sun, 1 Feb 2026
Fri, 25 Dec 2026	Fri, 3 Apr 2026	29 Mar - 8 Apr 2026	Fri, 20 Mar 2026
Fri, 1 Jan 2027	Fri, 10 Apr 2026	5-15 Apr 2026	Fri, 27 Mar 2026

Find the row closest to your due date, or enter your exact date in the tool above for your personal estimate.

What can you use a conception date calculator for?

Working back from a due date is useful in several situations:

• Curiosity and timeline — answering “when did I conceive?” once you already have a due date from a scan or another tool.
• Pinpointing a window — narrowing down which weekend or trip lines up with the likely conception window.
• Pregnancy dating — recovering your estimated LMP when you cannot remember it, since dating is measured from the LMP.
• Cross-checking — comparing the estimate against your own cycle and ovulation tracking for consistency.
• Counting days — for the raw gap between any two dates, the date difference calculator gives an exact day count.

Tips and common mistakes

A few points help you read the result correctly:

• Treat conception as a window, not one day. The plus or minus 5 days exists because ovulation timing and sperm survival vary.
• Use a confirmed due date if you have one. An early dating ultrasound gives a more reliable due date than an LMP-based guess, which makes the conception estimate better too.
• Do not confuse the LMP with conception. The LMP date is about 14 days before conception; the calculator shows both so they are not mixed up.
• Irregular cycles shift ovulation. If you ovulate earlier or later than day 14, your real conception date can sit outside the standard window.
• The 266-day figure is an average. Individual cycles vary, so the math is a starting estimate rather than a precise record.

Limitations and notes

This tool uses a single, widely taught dating rule: 266 days from due date to conception and 280 days to the LMP. It assumes ovulation about 14 days after the LMP and an average-length cycle. For very irregular cycles, IVF transfers (where the embryo age is known directly), or an uncertain due date, the estimate is less reliable than clinical dating. It cannot confirm the exact day of conception, and it does not assess the health of a pregnancy.

Your input stays in your browser. The calculation runs entirely on your device — nothing you type is uploaded or stored, so your dates remain private.

Medical disclaimer: This conception date calculator provides a general estimate for information only and is not medical advice. It is not a substitute for an examination, an ultrasound, or professional judgement. For any care decisions, or if you have questions about your pregnancy, please consult a doctor, midwife or qualified healthcare professional.

To plan or track from here, pair this with the due date calculator to work forward from your last period, the ovulation calculator to estimate your fertile window, and the pregnancy week calculator to follow each week, or browse all related tools on the pregnancy & women page.