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Gestational Age Calculator

Updated Sep 2026 Used 0 times
Choose the clinical date you know.
First day of the last menstrual period.
The documented estimated due date.
Date on which gestational age is estimated.

Guest calculations stay on this device. Signed-in results sync privately.

Your result

Gestational Age

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

Inputs usedReview the information used for this result.
Full calculation and sourcesGestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 days.

Versioned calculationFormula v1.0.0

Formula definitionUnit-normalized calculationFormula v1.0.0

What this result means

Calculate gestational weeks and days from an LMP or known due date to a selected reference date.

Method used

Gestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 days.

Important limitation

The calculation is an LMP-based estimate. Clinical care should use the documented best obstetric estimate, particularly when ultrasound or ART dating is available.

Result actions
1
Enter the inputsUse values with the units and definitions shown beside each field.
2
Apply the methodGestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 days.
3
Inspect the bridgeReview the primary result and supporting outputs before using it.
4
Check limitationsConfirm the assumptions, date, jurisdiction or population match your use case.

Interpretation

Clinical care should use the documented best obstetric estimate.

Gestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 days.

The calculation is an LMP-based estimate. Clinical care should use the documented best obstetric estimate, particularly when ultrasound or ART dating is available.

Use this result

Keep exploring

2 available tools
Pregnancy Due Date CalculatorEstimate a due date from LMP, conception, ultrasound dating, a known due date or IVF transfer with ACOG-aligned limitations.Conception CalculatorEstimate conception timing from LMP, due date, conception date, ultrasound dating or IVF transfer and show a wider intercourse window without claiming paternity evidence.
Public verification recordFormula v1.0.0
Review scope
Gestational Age Calculator Formula, Input Validation, Edge Behavior And Result Presentation.
Review team
CalculatorGeek Expert Review Team
Verified
2026-09-21
Next source review
2027-09-21
Automated fixtures
1 cases

Review method

Package reconciliation, primary-source mapping, competitor feature comparison, deterministic fixtures, PHP/JavaScript parity and responsive preview QA.

Known limitations

  • The calculation is an LMP-based estimate. Clinical care should use the documented best obstetric estimate, particularly when ultrasound or ART dating is available.
  • Changing legal, medical, exam or statistical conventions must be checked at the source.

Primary sources

Latest update

2026-09-21 ·

On this page

Direct answer

Calculate gestational weeks and days from an LMP or known due date to a selected reference date.

Continue with the Pregnancy Due Date Calculator when that method matches your next question.

Continue with the Conception Calculator when that method matches your next question.

Formula and methodology

Gestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 days.

Clinical care should use the documented best obstetric estimate.

Continue with Pregnancy Due Date Calculator or Conception Calculator for the next calculation.

Worked example

Clinical care should use the documented best obstetric estimate.

The calculator retains working precision and rounds only the displayed values.

Interpret the result carefully

Method boundary: The calculation is an LMP-based estimate. Clinical care should use the documented best obstetric estimate, particularly when ultrasound or ART dating is available.

Inputs, boundary behavior and JavaScript/PHP parity are covered by deterministic release tests.

Use the result for the named task only. Do not stretch it into a legal, clinical, safety or causal conclusion the formula does not support.

Inputs, outputs and result meaning

Calculates gestational age, additional days, total gestational days, and estimated due date from calculate from, first day of LMP, known due date, and as of date with visible method, validation, precision, and limitations.

Inputs

Calculate from
Choose the clinical date you know. Available choices are Last menstrual period and Known due date.
First day of LMP
First day of the last menstrual period. The supported range is 1900-01-01 through 2100-12-31.
Known due date
The documented estimated due date. The supported range is 1900-01-01 through 2100-12-31.
As of date
Date on which gestational age is estimated. The supported range is 1900-01-01 through 2100-12-31.

Outputs

Gestational age
This is the primary result. It is displayed in weeks. The visible value is displayed without decimal places.
Additional days
It is displayed in days. The visible value is displayed without decimal places.
Total gestational days
It is displayed in days. 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.

Frequently asked questions

What method does this calculator use?

Gestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 days.

How should I round the result?

Keep full precision during the calculation and round only the final display unless a governing standard says otherwise.

What is the main limitation?

The calculation is an LMP-based estimate. Clinical care should use the documented best obstetric estimate, particularly when ultrasound or ART dating is available.

Can I reproduce the answer?

Yes. The formula, inputs, supporting outputs and worked example are displayed on the page.

Sources

Source research refreshed September 21, 2026. No rankings, review credentials or official affiliation are implied.