What this result means
Calculate gestational weeks and days from an LMP or known due date to a selected reference date.
Gestational Age Calculator
Use the Gestational Age Calculator to calculate and interpret gestational age.
Gestational Age
Enter your values, then calculate to see a verified result.
Reference range
How this result is calculated
A clear calculation path based on your inputs.
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 v1.0.1
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.
Enter the inputsUse values with the units and definitions shown beside each field.
Apply the methodGestational age counts elapsed time from the LMP-equivalent date; a known due date maps back 280 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
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
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.
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Report a calculation issue Gestational Age Calculator: purpose and method
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.
Keep exploring
Related calculators and resources
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.
Method and test recordFormula v1.0.1
- Recorded scope
- Gestational Age 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 SERP, observed AI Overview and PAA reconciliation; current ACOG/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
- The calculation is an LMP-based estimate. Clinical care should use the documented best obstetric estimate, particularly when ultrasound or ART dating is available.
- Calendar months are not a precise substitute for gestational weeks and days.
- This date calculator does not interpret ultrasound biometry such as BPD, HC, AC, or FL.
- Changing legal, medical, exam or statistical conventions must be checked at the source.
Method sources
- ACOG — Methods for Estimating the Due Date
- ACOG — How Your Fetus Grows During Pregnancy
- NHS — Pregnancy due date calculator
Latest model update
2026-11-03 - Added evidence-backed gestational-age answers for the observed PAA set, strengthened ACOG dating boundaries, replaced off-intent sources, and preserved the calculation contract.
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; Known due date: 2027-03-08; As of date: 2026-09-21 | Gestational age: 16 weeks; Additional days: 0 days; Total gestational days: 112 days; 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
Gestational age in weeks and days
Gestational age is the elapsed time from the LMP-equivalent date to the selected as-of date, shown as completed weeks plus additional days.
When a documented due date is used, this calculator maps back 280 days and then counts forward. It is an informational date estimate, not a replacement for the best obstetric estimate recorded by a clinician.
Need to estimate the due date first? Use the Pregnancy Due Date Calculator.
For a separate timing estimate, open the Conception Calculator.
Formula and methodology
LMP method: gestational age = as-of date − first day of LMP
Known due date method: LMP-equivalent date = due date − 280 days
The total day count is divided into completed seven-day weeks plus the remaining days. ACOG states that the LMP, the first accurate ultrasound, or both should be used to establish and document the estimated due date. First-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age; ART pregnancies require ART-specific dating.
Worked example
With an LMP of June 1, 2026 and an as-of date of September 21, 2026, 112 days have elapsed.
112 ÷ 7 = 16 completed weeks and 0 additional days. Adding 280 days to the LMP gives an estimated due date of March 8, 2027.
The calendar arithmetic retains exact dates; the visible answer separates completed weeks from additional days.
Interpret the result carefully
Use the result as a date estimate. ACOG recommends documenting the estimated due date when LMP and/or the first accurate ultrasound information becomes available. Subsequent changes should be uncommon and clinically documented.
- An uncertain LMP, irregular cycles, assisted reproduction, or a dating-ultrasound discrepancy can change the clinical estimate.
- Calendar months vary in length, so pregnancy is tracked more precisely in weeks and days.
- This tool does not interpret ultrasound measurements such as biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), or femur length (FL). Those measurements require the relevant chart, scan context, and clinician interpretation.
Inputs, boundary behavior, and JavaScript/PHP parity remain covered by deterministic release tests.
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
How to calculate gestational age?
Count the elapsed days from the first day of the last menstrual period to the selected date, then express the total as completed weeks plus remaining days. If only a documented due date is known, this tool uses the LMP-equivalent date 280 days before that due date. Clinical dating may instead rely on ultrasound or ART information.
What is the normal range for BPD, HC, AC, and FL at 32 weeks of pregnancy?
This date calculator cannot supply or interpret a single normal range for fetal biometry. BPD, HC, AC, and FL must be plotted on the appropriate ultrasound growth reference and interpreted with scan quality, gestational dating, and the clinical record.
How many weeks is 6 months pregnant?
There is no exact conversion because calendar months have different lengths. Pregnancy care therefore uses completed weeks and days; use a dated LMP or documented due date rather than a month label when precision matters.
Is due date based on 39 or 40 weeks?
The conventional estimated due date is 40 weeks, or 280 days, from the first day of the LMP. That date is an estimate rather than a promise of the delivery day.
Sources
- ACOG — Methods for Estimating the Due Date
- ACOG — How Your Fetus Grows During Pregnancy
- NHS — Pregnancy due date calculator
Sources and saved U.S. search evidence were reviewed November 3, 2026. The page does not claim to interpret ultrasound biometry or replace individualized obstetric dating.