Conceive Calculator
Conception Date Calculator works out Probable date of conception and That was a from Work back from, Estimated due date and Date of birth. The default figures on this page give probable date of conception of 2026-01-15; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- The estimated due date is 40 0/7 weeks, and conception sits 14 days after the dating LMP, so it is 266 days before the due date.
- 2026-10-08 - 266 days = 2026-01-15.
- The fertile window is the six days ending on ovulation, so 2026-01-10 to 2026-01-15.
- Implantation follows 6-12 days later.
- Working forward again, conception + 266 days gives a due date of 2026-10-08.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the Conception Date Calculator calculates
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed.
How to use this Conception Date Calculator
Enter Work back from, Estimated due date, Date of birth and Gestational age at birth (weeks). Probable date of conception updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Probable date of conception, That was a and Intercourse window opens.
The method follows ACOG Committee Opinion No. 700: Methods for Estimating the Due Date, Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle and Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy.
Due 8 October 2026 example
With Work back from: due-date, Estimated due date: 2026-10-08, the calculator returns Probable date of conception: 2026-01-15, Implied last menstrual period: 2026-01-01, Intercourse window opens: 2026-01-10. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Probable date of conception, That was a and Intercourse window opens. The inputs you can vary are Work back from, Estimated due date, Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days) and First day of the last menstrual period.
Understanding Probable date of conception, That was a and Intercourse window opens
An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
Can I calculate my due date based on the day of conception?
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed. Open “How this is worked out” beside the result to see each step for your own figures.
Can you tell the day you conceive?
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed. Put your own figures in above and the working updates beside the result. For scale, the default figures on this page give 2026-01-15.
Are you already 2 weeks pregnant at conception?
It needs Work back from, Estimated due date, Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days) and First day of the last menstrual period. Nothing else is required, and the figures stay in your browser.
Am I 4 weeks pregnant if I conceived 2 weeks ago?
The result gives Probable date of conception, That was a, Intercourse window opens, Intercourse window closes, Intercourse window and Implantation window opens, each labelled with its unit so the figure can be read on its own.
What information do I need for the Conception Date Calculator?
You need Work back from, Estimated due date, Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days), First day of the last menstrual period, Average cycle length and Luteal phase length. The page opens with default figures that give probable date of conception of 2026-01-15, so you can see the scale before entering your own.
What does this Conception Date Calculator show?
It shows Probable date of conception, That was a, Intercourse window opens, Intercourse window closes, Intercourse window, Implantation window opens, Implantation window closes and Implied last menstrual period, with the working and any warnings for your entries.