Conception Calculator

Updated Oct 2026 Used 5 times

Use the Conception Calculator to calculate and interpret estimated conception.

Your result

Conception

Enter your values, then calculate to see a verified result.

Inputs usedReview the information used for this result.

Full calculation and sourcesLMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Versioned calculationFormula v1.0.1

What this result means

Estimate conception timing from LMP, due date, conception date, ultrasound dating or IVF transfer and show a wider intercourse window without claiming paternity evidence.

Method used

LMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Important limitation

Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.

Enter the inputsUse values with the units and definitions shown beside each field.

Apply the methodLMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Inspect the bridgeReview the primary result and supporting outputs before using it.

Check limitationsConfirm the assumptions, date, jurisdiction or population match your use case.

Interpretation

A date estimate cannot prove which intercourse caused pregnancy or establish paternity.

LMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.

Use this result

Pregnancy Due Date CalculatorEstimate the due date from the dating input you know.Gestational Age CalculatorCalculate weeks and days from LMP or a documented due date.Basal Body Temperature Shift CalculatorCompare consistent temperature averages without confirming ovulation.

Conception Calculator: purpose and method

Calculates estimated conception, possible intercourse-window start, possible intercourse-window end, and estimated due date from calculate from, first day of LMP, estimated due date, and known conception date with visible method, validation, precision, and limitations.

Keep exploring

4 related pages

Method and test recordFormula v1.0.1

Recorded scope: Conception Calculator formula, input validation, edge behavior and result presentation.
Publisher: CalculatorGeek
Reviewed by: CalculatorGeek Editorial Team
Recorded review date: 2026-11-03
Next source review: 2027-05-03
Definition fixtures: 1 configured scenarios
Published examples: 1 shown below

Recorded method

Saved five-result U.S. English SERP, observed AI Overview and competitor-page reconciliation; current ACOG/ASRM/NHS source review; deterministic fixtures; PHP/JavaScript parity; and responsive preview QA.

These records describe the published model and reference tests. A test-case count is not a certification of every possible input or an independent specialist review. Editorial policy

Known limitations

Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.
Changing legal, medical, exam or statistical conventions must be checked at the source.

Method sources

Latest model update

2026-09-21 -

Reference inputs and expected results

Up to 12 examples from the configured definition fixtures are shown. Expected values use the stated output units; invalid inputs are intended to be rejected.

Case	Inputs	Expected result
Primary worked example	Calculate from: lmp; First day of LMP: 2026-06-01; Estimated due date: 2027-03-08; Known conception date: 2026-06-15; Ultrasound date: 2026-08-24; Gestational weeks on scan: 12 value; Additional gestational days: 0 value; Embryo transfer date: 2026-06-18; Typical cycle length: 28 value	Estimated conception: 20260615; Estimated due date: 20270308 (allowed numeric tolerance: 0.0001)

Report an issue with this tool. Include the page URL, units, expected answer, and steps to reproduce. Do not include sensitive personal information.

On this page

What a conception date calculator can estimate

A conception date calculator estimates when fertilization may have occurred; it cannot identify an exact intercourse date. With a 28-day LMP model, the center estimate is 14 days after the first day of the last menstrual period. From an established due date, it is 266 days earlier. Ultrasound and IVF modes use the recorded dating inputs instead of pretending every pregnancy follows one calendar pattern.

Use the conception date versus intercourse date guide to understand why the displayed window is broader than one day.

For the documented pregnancy timeline, continue with the Pregnancy Due Date Calculator or Gestational Age Calculator.

Conception-date formulas and dating methods

LMP-based due date = LMP + 280 days + (cycle length - 28 days)

Estimated conception = estimated due date - 266 days

Possible intercourse window = center estimate - 5 days through + 1 day

A known conception date maps forward 266 days. Ultrasound mode adds the remaining days in a 280-day gestational model to the scan date. IVF mode adds 263 days after a day-3 transfer or 261 days after a day-5 transfer, matching the embryo-age convention described by ACOG.

The displayed intercourse window preserves uncertainty; it is not a timestamp, ovulation confirmation, paternity result or fertility diagnosis.

Worked conception-date example

For an LMP of June 1, 2026, a 28-day cycle adds 280 days for an estimated due date of March 8, 2027. Counting back 266 days gives an estimated conception date of June 15, 2026.

The calculator displays a possible intercourse window from June 10 through June 16. That range does not prove which intercourse led to pregnancy, because ovulation, fertilization and sperm survival cannot be reconstructed exactly from calendar dates.

How to interpret the estimate

Calendar dating is an estimate. Cycle length and ovulation timing vary, even when cycles appear regular. ACOG identifies first-trimester ultrasound as the most accurate method to establish or confirm gestational age and recommends ART-derived dating when pregnancy resulted from assisted reproductive technology.

Keep the clinician-documented best obstetric estimate as the clinical reference. This calculator cannot identify a precise fertilization event, establish paternity or replace ultrasound, fertility-clinic records or prenatal care.

Inputs, outputs and result meaning

Inputs

Calculate from: Choose the date and dating method you know. Available choices are Last menstrual period, Estimated due date, Known conception date, Ultrasound gestational age, IVF day-3 embryo transfer, and IVF day-5 embryo transfer.
First day of LMP: Used in LMP mode. The supported range is 1900-01-01 through 2100-12-31.
Estimated due date: Used in due-date mode. The supported range is 1900-01-01 through 2100-12-31.
Known conception date: Used when conception is known clinically. The supported range is 1900-01-01 through 2100-12-31.
Ultrasound date: Date of the scan used for dating. The supported range is 1900-01-01 through 2100-12-31.
Gestational weeks on scan: Completed gestational weeks recorded at the scan. The supported range is 0 through 42.
Additional gestational days: Additional days after completed weeks. The supported range is 0 through 6.
Embryo transfer date: Use the transfer date recorded by the fertility clinic. The supported range is 1900-01-01 through 2100-12-31.
Typical cycle length: Used only to adjust LMP dating. The supported range is 21 through 45.

Outputs

Estimated conception: This is the primary result. The visible value is displayed without decimal places.
Possible intercourse-window start: The visible value is displayed without decimal places.
Possible intercourse-window end: The visible value is displayed without decimal places.
Estimated due date: The visible value is displayed without decimal places.

The calculator applies the documented method after validating and normalizing the inputs. Keep the result label, unit, selected mode, and stated limitations together when sharing the answer.

Questions about conception-date estimates

Is the conception date the same as the intercourse date?

Not necessarily. Sperm may remain capable of fertilization for several days, and the calendar estimate cannot identify one causal intercourse date.

How is conception estimated from a due date?

The calculator subtracts 266 days from the established due date, then shows a wider possible intercourse window around that center estimate.

Does a regular cycle make the date exact?

No. The LMP method adjusts for the entered typical cycle length, but actual ovulation can still vary from cycle to cycle.

How does IVF mode calculate the date?

It uses the recorded transfer date and embryo age. The due-date model adds 263 days after a day-3 transfer or 261 days after a day-5 transfer; the clinic’s documented date remains the clinical reference.

Sources and evidence limits

Sources and calculation boundaries were checked November 3, 2026. The cited organizations did not review or endorse this CalculatorGeek page. The result is an informational estimate, not a diagnosis or paternity determination.