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Conception Date Calculator

Use this conception date calculator to estimate conception timing from an estimated pregnancy due date.

Method and assumptions documentedPlanning estimateSource note includedNo expert review claimed

Live calculator

Conception date

Estimated conception dateMay 15, 2026

Estimated due date minus 266 days.

Conception windowMay 12, 2026 - May 18, 2026

A rough +/- 3 day window around the estimate.

Estimated LMPMay 1, 2026

Estimated due date minus 280 days.

Use this as a planning estimate, not medical advice. Medical conditions, medication, pregnancy, eating disorder history, or disease-specific nutrition needs should be handled with a qualified professional.

Quick answer

Conception Date Calculator: what it calculates

Conception Date Calculator calculates estimated conception date from estimated due date. The visible formula is Estimated conception date = estimated due date - 266 days.

ResultEstimated conception date
InputsEstimated due date
FormulaConception date estimate

Formula

Conception date estimate

Estimated conception date = estimated due date - 266 days

This is an estimate. Conception timing can vary, and clinical dating may use ultrasound or other information.

How to use

Steps

  1. Enter the estimated due date.
  2. Review the estimated conception date.
  3. Use the window for rough timing, not certainty.
  4. Confirm pregnancy dating with a qualified clinician.

Example

Sample calculation

Due dateFeb 5, 2027
Estimated conceptionMay 15, 2026
Estimated LMPMay 1, 2026

Calculator use

Best for

  • Use this conception date calculator to estimate conception timing from an estimated pregnancy due date.
  • Estimating due date, pregnancy week, trimester, or conception timing for planning conversations.
  • Checking date assumptions from last menstrual period, due date, or cycle length with the method visible.
  • Preparing non-diagnostic timing context before confirming dates with a qualified clinician.

Before relying on it

Check first

  • Treating pregnancy date estimates as diagnosis, medical advice, or a substitute for clinician dating.
  • Ignoring cycle variability, ultrasound timing, fertility treatment dates, irregular periods, or clinician guidance.
  • Using late or uncertain dates as if they were exact conception or due-date proof.

Details

What to know before using the result

Estimate basisDue date minus 266 days

This approximates fertilization timing from a 40-week pregnancy estimate.

Window, not certainty+/- 3 days shown

Ovulation, sperm survival, implantation, and clinical dating uncertainty make exact conception dates hard to determine.

Clinical contextConfirm dates

Use clinician-provided dating for medical, legal, or family-planning decisions.

Source notes

References used for this calculator

Cleveland ClinicExplains that one way to estimate conception from due date is subtracting 266 days.ACOGProvides clinical guidance on due-date estimation methods and gestational dating.

Benchmarks

How to read the result

Due date - 266 days: Conception estimate.

A common back-calculation from estimated due date.

Due date - 280 days: LMP estimate.

Approximate first day of last menstrual period for a 40-week estimate.

+/- several days: Expected uncertainty.

Exact conception timing is usually not knowable from a due date alone.

Calculator accuracy

Methodology and assumptions

Formula

Estimated conception date = estimated due date - 266 days

Inputs used

Estimated due date

Limitations

Health results are screening or planning estimates. Age, body composition, medication, medical history, pregnancy, and professional guidance can change the right interpretation.

Review record

Method and sources checked on July 12, 2026. The documented method, source timing, validity conditions, and reliance limits were checked on this date.

Method provenanceEstimate · toolkitshelf.conception-from-edd.v1
Maintained by
Toolkit Shelf
Calculation class
Estimate
Method version
toolkitshelf.conception-from-edd.v1
Source date
May 2017; reaffirmed 2025
Source checked
July 12, 2026
Intended audience
People making a rough conception-date estimate from an already established estimated due date.
Valid when
The entered due date must already be a reliable obstetric estimate. Ultrasound, assisted reproduction, irregular cycles, and clinician-assigned dating take precedence over this 266-day subtraction.
Reliance boundary
The result is calendar back-calculation, not proof of conception, ovulation, intercourse, parentage, clinical gestational dating, or a fertile-window determination.
Data freshness
ACOG's due-date framework and clinical dating limitations were checked on July 12, 2026. The plus-or-minus three-day window is a visible Toolkit Shelf planning convention.
Cite this page

Toolkit Shelf. Conception Date Calculator. https://toolkitshelf.com/tools/conception-date-calculator

FAQ

Common questions

How do I calculate conception date from due date?

A common estimate subtracts 266 days from the estimated due date.

Is conception date exact?

No. It is an estimate because ovulation timing, fertilization timing, and clinical dating can vary.

Why is LMP different from conception date?

Pregnancy dating is commonly counted from the last menstrual period, which is usually about two weeks before conception.

When should I ask a clinician?

Ask a clinician or qualified nutrition professional when pregnancy, medication, medical history, eating disorder risk, symptoms, or disease-specific guidance matters.

Why can health formulas be misleading?

Health formulas simplify age, sex, body composition, training status, measurement method, pregnancy, and medical context into a planning estimate.

Is this medical advice?

No. Health calculators are screening or planning tools and do not diagnose, treat, or replace professional medical or nutrition guidance.