CalculationTime

Conception Date Calculator

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Your numbers, formula and explanation together

Conception Date Calculator: 2026-01-08. due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01

Formula applied

The exact method behind this answer

CalculationTime keeps the method visible so the number can be checked instead of blindly trusted.

From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.
  1. Apply the formulaFrom due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.2026-01-08due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01

Your live breakdown

Current inputs in the calculation

These values come from the controls above and update when the calculator changes.

Calculation method
Due date
Choose the date basis you have.
Known date year
2,026
Due date, LMP date, or ultrasound scan date year.
Known date month
10
Month number for the known date.
Known date day
1
Day of month for the known date.
Ultrasound gestational weeks
8 weeks
Use only for ultrasound method.
Extra gestational days
0 days
Use only for ultrasound method.

Resulting answer

2026-01-08

due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01

Answer
2026-01-08
Live support
due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01

Assumptions used

What this answer assumes

Pregnancy-date orientation only, not diagnosis, contraception, fertility treatment, legal proof or medical advice.

  • The estimate is not proof of biological parentage or exact conception timing.
  • Ovulation, implantation and dating can vary by cycle length, health context and scan timing.
  • Medical dating should be confirmed with a qualified clinician.
  • Ultrasound gestational age is treated as completed weeks plus days on the scan date.

Master’s Tip

How to use the result well

Master’s Tip: treat this as a date window, not a courtroom answer. Pregnancy dating is strongest when LMP, scan results and clinician assessment agree.

Printable record

What belongs in the saved calculation

Save the inputs, result, formula, assumptions, page URL and date together so the calculation can be reviewed later.

Calculation method
Due date
Choose the date basis you have.
Known date year
2,026
Due date, LMP date, or ultrasound scan date year.
Known date month
10
Month number for the known date.
Known date day
1
Day of month for the known date.

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Direct answer

Conception Date Calculator in one sentence

A conception date calculator estimates ovulation/conception from a due date, last menstrual period or ultrasound gestational age. It uses ordinary obstetric dating arithmetic and keeps the uncertainty visible.

How to use this calculator

  1. Enter calculation method, known date year, known date month, known date day and the remaining visible inputs.
  2. Check the formula, assumptions and worked example before reusing the number.
  3. Use the result, related calculators and printable record as the next practical step.

Default result preview

Estimated conception date: 2026-01-08

This pre-calculated state lets readers verify what the tool returns before the inputs change.

Show the working

From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.

The default inputs, formula and result stay together so the number can be checked or quoted without losing context.

Export this result

Copy the default solved state as Markdown or CSV, then print the page for a clean formula and result record.

Print or save report: use the browser print command to save the visible formula, result, assumptions and source context as a clean PDF.

Explain it like I'm 12

A conception date calculator estimates ovulation/conception from a due date, last menstrual period or ultrasound gestational age. It uses ordinary obstetric dating arithmetic and keeps the uncertainty visible.

Formula

From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.

Worked example

If the due date is 1 October 2026, estimated conception is about 266 days earlier, with a fertile-window estimate around the five days before that date through ovulation.

Professional note

Master’s Tip: treat this as a date window, not a courtroom answer. Pregnancy dating is strongest when LMP, scan results and clinician assessment agree.

Regional and unit assumptions

General obstetric dating arithmetic: due date about 280 days after LMP and conception about 266 days before due date.

Assumptions and limitations

Methodology & Accuracy

How this calculator is checked

CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.

Formula used

From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.

Standard or basis

General obstetric dating arithmetic: due date about 280 days after LMP and conception about 266 days before due date.

Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.

Master's Tip

Master’s Tip: treat this as a date window, not a courtroom answer. Pregnancy dating is strongest when LMP, scan results and clinician assessment agree.

Authority & freshness

Who checked this calculator?

Page structure checked: 2026-09-26. Calculator-specific statutory or source-table dates appear in the revision log when the tool depends on time-sensitive rules.

Published by CalculationTime

CalculationTime publishes calculator pages with visible formulas, assumptions, worked examples, related next steps and printable records so the result can be audited instead of treated as a black box.

Machine-readable formula

From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.Formula text is also exposed in the page schema and visible methodology block.

Source basis

2 source references are attached to this page.

Related calculators

These cards are resolved from calculator data, category context and live-route status so the page links to useful next steps without hand-built link clutter.

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Knowledge check

Test your understanding

Use these quick checks to confirm that the result, formula and assumptions make sense before you reuse the number.

Which inputs drive the default result?

The default result starts with Calculation method, Known date year, Known date month. The current pre-solved output is estimated conception date = 2026-01-08.

Where is the calculation proof?

The proof is in the formula, worked example and assumptions sections. Together they show the arithmetic, the default state and the limits of the result.

What should you do after reading the answer?

Use the related calculators, printable record or source notes to check the next practical step instead of treating one output as the end of the workflow.

Accuracy feedback

Did this calculator work accurately?

This lightweight check records your answer in this browser only. It does not send personal data and does not claim a live backend review queue.

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Questions

Can conception date be exact?

Usually no. Fertilization, ovulation timing and implantation vary, so a window is safer than a single exact date.

Why subtract 266 days from due date?

A common pregnancy due date is about 280 days from LMP, and ovulation/conception is often estimated about 14 days after LMP.

Can this identify a father?

No. Use medical and legal advice for paternity questions.

Calculation note

Pregnancy dates are conventionally measured from LMP even though conception happens later. A useful calculator names that convention and its uncertainty.

Sources and further readingACOG — Methods for Estimating the Due Date