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Conception Calculator

Health

Estimate your likely conception date range from your last menstrual period and cycle length, or work backward from a known due date. Two calculation modes.

What Do You Know?
26th May 2026
Average Cycle Length
days
20 days45 days

Likely Conception Date

Enter your dates above

Informational and educational use only — not a substitute for guidance from your OB/GYN, especially if conception timing has legal or clinical significance.

What is a Conception?

A Conception Calculator estimates the date range during which you most likely conceived, using either your last menstrual period (LMP) and average cycle length, or working backward from a known due date. Both methods rely on the well-established relationship between ovulation timing and a standard 14-day luteal phase — the consistent gap between ovulation and the start of the next expected period.

This calculator supports two input modes so you can use whichever information you already have. If you know your LMP and average cycle length, conception is estimated as LMP + (cycle length − 14 days) — essentially your estimated ovulation day, since conception happens at or very near ovulation. If you instead only know your due date, conception is estimated as the due date minus 266 days, which is mathematically equivalent to 38 weeks before delivery.

Because ovulation timing naturally varies and sperm can survive several days in the reproductive tract, the calculator shows a realistic date range around its central estimate rather than a single falsely precise day. This is informational and educational content only, not personalized medical or legal advice.

How to use this Conception calculator

  1. Choose your input mode using the toggle — "My last period date" or "My due date" — depending on which one you already know.

  2. If using LMP mode, enter the first day of your last period and your average cycle length.

  3. If using due date mode, enter your estimated or confirmed due date.

  4. Read your Likely Conception Date — the large date in the result card.

  5. Note the Likely Range shown beneath it, reflecting the natural few-day uncertainty in ovulation and conception timing.

  6. Cross-reference with the EDD Calculator if you want to verify the due-date relationship, or the Pregnancy Due Date Calculator for a fuller due-date breakdown with trimester progress.

  7. Use your fertility clinic's specific dates instead if you conceived via IVF or a monitored insemination cycle, since those are far more precise than this calendar-based estimate.

Formula & Methodology

Mode A: From LMP + Cycle Length
Conception Date = LMP Date + (Cycle Length − 14) days

Mode B: From Due Date
Conception Date = Due Date − 266 days   (equivalent to 38 weeks before delivery)

Likely Range
Range Start = Conception Date − 2 days Range End   = Conception Date + 2 days

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Worked Example (LMP mode):

A woman's last period started on a given date, and her average cycle length is 30 days.

| Variable | Value |
|---|---|
| LMP Date | Day 1 |
| Cycle Length | 30 days |

Conception Date = LMP + (30 − 14) = LMP + 16 days

If her LMP was March 1st, her estimated conception date is March 17th, with a likely range of March 15th to March 19th.

Worked Example (Due Date mode):

A different patient's provider has confirmed a due date of December 1st.

Conception Date = December 1st − 266 days = March 10th, with a likely range of March 8th to March 12th.

Both approaches converge on mid-March as the likely conception window — useful context for understanding early pregnancy timing. See the Crown-Rump Length Calculator if you've had a dating ultrasound and want to cross-check gestational age against these estimates.

Frequently Asked Questions

Conception date is estimated as the first day of your last period plus your average cycle length minus 14 days, which corresponds to your estimated ovulation day. This works because the luteal phase — the time between ovulation and the start of the next period — is consistently about 14 days for most people, regardless of overall cycle length, while conception itself happens at or very close to ovulation.
Working backward from a due date calculated using Naegele's Rule (LMP + 280 days), conception is estimated as the due date minus 266 days — that is, 280 days minus the same 14-day luteal phase adjustment used in the LMP-based method. This is equivalent to 38 weeks before the due date, since conception occurs roughly 2 weeks after the last menstrual period in a standard cycle.
Conception can only be pinpointed to within a few days even with precise cycle tracking, because ovulation timing itself naturally varies cycle to cycle and sperm can survive in the reproductive tract for up to 5 days before fertilizing an egg. The calculator shows a 2-day range on either side of its best estimate to reflect this natural uncertainty honestly, rather than implying false precision.
Use the LMP mode if you know the first day of your last period and your average cycle length, which is generally the more direct calculation. Use the due date mode if you only know your estimated or confirmed due date from an ultrasound or your provider, and want to work backward to estimate when conception likely occurred.
No. This calculator provides a general estimate for informational purposes only. Legal contexts such as paternity determinations typically require more precise methods, including genetic testing, and should never rely on a calendar-based estimate like this one. Please consult appropriate legal and medical professionals for any situation with legal implications.
Yes. Both calculation modes assume a fairly typical ovulatory cycle with a standard ~14-day luteal phase. If your cycles are notably irregular, or if you conceived via fertility treatment with a known trigger or insemination date, that specific date is a more reliable indicator of conception timing than this calendar-based estimate.
Conception date and due date are mathematically linked: due date = conception date + 266 days (38 weeks), or equivalently, due date = LMP + 280 days (40 weeks) under Naegele's Rule. This calculator lets you move between the two starting points, depending on which one you already know.
If you know your exact embryo transfer date and embryo age (day 3 or day 5), your clinic's own conception date calculation from that specific procedure will be far more precise than this calendar-based estimate, which assumes a standard natural-cycle ovulation timing. Use your clinic's provided dates for IVF pregnancies.
The EDD Calculator works forward from your LMP to estimate your due date using Naegele's Rule alone. This Conception Calculator works in the reverse direction (or from LMP directly) to estimate when conception itself likely occurred, which is a related but distinct question — useful for understanding early pregnancy timing rather than delivery timing.
Choose whether you know your last period date or your due date using the toggle at the top. Then enter the relevant date (and cycle length, if using LMP mode). The calculator instantly shows your estimated conception date and a realistic 4-day range around it.
No. All calculations happen entirely within your browser. None of the dates you enter are transmitted to or stored on any server, and everything is cleared when you close the page.
Also known as
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