Calculate the estimated due date and LMP implied by a current gestational week and day. The calculator runs entirely in your browser and pairs a focused answer with the method, assumptions and the related tools worth checking next.
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Enter the completed gestational weeks and extra days on a date you choose. The calculator counts back that many days to the implied first day of the last menstrual period, then adds the standard 280 days of the 40-week model to give the estimated due date, with the implied LMP and days remaining shown alongside.
Gestational age is conventionally counted from the first day of the last menstrual period (LMP), so the tool recovers that date first: reference date minus (weeks × 7 + days). The due date is then the LMP plus 280 days — the standard model behind LMP-based due dates.
Counting back 143 days gives an implied LMP of 30 May 2026, and 280 days forward from there gives an estimated due date of Saturday 6 March 2027 — 137 days away, in the second trimester. Edge case: enter 40 weeks 0 days and the due date is the reference date itself, because all 280 days of the model are already complete.
The 280-day model takes no account of cycle length, the actual conception date or ultrasound measurements, and real pregnancies rarely run exactly 40 weeks. Clinical dating and scan measurements take precedence over this estimate.
Enter the completed gestational weeks and days to work back to the implied dates. Each step below is a short action rather than a wall of explanation.
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Work backwards to the implied last menstrual period, then apply the standard 280-day model forward.
Similar tools answer different questions. Match the inputs and the reference point to the answer you actually need.
The numbers come from your inputs; this shows the shape of the calculation.
Enter the completed weeks of gestation reached on the date you are measuring from.
Enter the extra days beyond the completed weeks, and the date you are measuring on under On this date.
The implied dates: 20 weeks 3 days on 20 October 2026 counts back 143 days to an implied LMP of 30 May 2026, and 280 days forward from there gives an estimated due date of Saturday 6 March 2027. A standard model, not a clinical date.
Use exact source inputs, keep the reference date visible and verify any official rule separately.
Set the reference date to the event that actually controls the decision, not to today.
Compare the stated method here against the other tool's convention before assuming an error.
For a neighbouring calculation, open the Gestational Age Calculator or the Pregnancy Due Date Calculator. To understand the reasoning first, read Corrected Age for Premature Babies: A Parent-Friendly Guide.
Choose another focused tool when the underlying question changes.
Understand the calendar rules, edge cases and next steps behind this calculator.
Answers about method, inputs, privacy, boundaries and responsible interpretation.
You enter the completed gestational weeks and extra days on a date you choose, and the calculator counts back that many days to the implied first day of the last menstrual period (LMP), then adds the standard 280 days to give the estimated due date, with the implied LMP and the days remaining shown alongside.
Because gestational age is conventionally counted from the first day of the last menstrual period, that is the date the model needs: the reference date minus the completed weeks multiplied by 7, plus the extra days. The due date is then simply the LMP plus 280 days.
At 20 weeks, 3 days on 20 October 2026, counting back 143 days gives an implied LMP of 30 May 2026, and 280 days forward from there gives an estimated due date of Saturday 6 March 2027, which is 137 days away, in the second trimester.
The due date comes out as the reference date itself, because all 280 days of the standard model are already complete.
Treat it as a standard-model estimate. The 280-day model takes no account of cycle length, the actual conception date or ultrasound measurements, and real pregnancies rarely run exactly 40 weeks. Clinical dating and scan measurements take precedence over this estimate.
Navjeet Kamboj is the author and editor of Age Calculator Lab. He focuses on making age, date, milestone and planning calculations easier to understand through clearly explained methods, practical examples and responsible limitations.