WorldCalcs

Conception Calculator

When conception likely happened, plus the fertile window and the matching due date.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

Estimated conception date

15 January 2026

Fertile window: 10 January 2026 → 15 January 2026

Estimated implantation window: 21 January 2026 → 27 January 2026

Corresponding due date: 8 October 2026

This tool provides general estimates for information only and is not medical advice. Always confirm dates and targets with your doctor or midwife. See our full disclaimer.

What a conception calculator does

A conception calculator estimates when conception most likely happened — or is happening — plus the fertile window around it. It's useful for pinning down a rough conception date from your last period, your due date, or a baby's birth date.

How conception is estimated

Ovulation usually happens about 14 days before the next period, so from your last period the estimate is LMP + (cycle length − 14). Working backwards from a due date, conception is due date − 266 days (38 weeks). From a birth date the same 266-day subtraction gives a rough estimate, but actual pregnancy length varies by about two weeks.

Example

If your last period began on January 1 and your cycle is 28 days, ovulation and conception are estimated around January 15, with a fertile window of about January 10 to 15. That points to a due date of October 8.

Related: Pregnancy Calculator, Ovulation Calculator, Due Date Calculator.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Pick how you want to calculate, then enter the date. The tool returns the estimated conception date, the fertile window (about the five days before ovulation plus ovulation day), the likely implantation window, and — where relevant — the corresponding due date.

Frequently asked questions

How do you estimate the date of conception?+

Conception happens around ovulation, roughly 14 days before your next expected period. From your last period, that is the start date plus (cycle length minus 14). From a due date, it is the due date minus 266 days.

Can I find the conception date from my due date?+

Yes, subtract 266 days (38 weeks) from the due date. That is the standard way to work backwards to the likely conception window.

Can I estimate conception from my baby's birth date?+

You can get a rough estimate by subtracting 266 days from the birth date, but actual pregnancy length varies by a couple of weeks, so treat it as an approximation rather than an exact date.

What is the fertile window?+

It is roughly the five days before ovulation plus ovulation day. Sperm can survive up to about five days, so intercourse in that window can lead to conception.

When does ovulation happen in my cycle?+

For most people ovulation is about 14 days before the next period. On a 28-day cycle that is around day 14; on a 32-day cycle it is around day 18. A longer or shorter cycle moves ovulation accordingly.

Is the conception date exact?+

No. It is an estimate based on average ovulation timing. Cycle-to-cycle variation, irregular cycles and the sperm's survival time all mean the true date can differ by a few days.

When does implantation happen?+

Implantation usually occurs about 6 to 12 days after conception, when the fertilized egg attaches to the lining of the uterus.

Does my cycle length change the result?+

Yes. Because ovulation is tied to when your next period would start, a longer cycle pushes conception later and a shorter cycle pulls it earlier.

References

  • American College of Obstetricians and Gynecologists (ACOG) — estimating gestational age and the estimated due date.

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