Due Date Calculator
Estimate your pregnancy due date from your last menstrual period
100% client-side processing
Key Milestones
This tool provides an estimate only and is not medical advice. Please consult your doctor or midwife to confirm your due date.
Estimate your pregnancy due date with Naegele’s rule — 280 days from the first day of your last menstrual period. See your current week of pregnancy, which trimester you are in, and key milestone dates like viability and full term. Everything runs locally in your browser, and the result is an estimate to discuss with your doctor, not a medical diagnosis.
How to Use
- Enter your LMP: Pick the first day of your last menstrual period in the date field.
- Optionally set the reference date: The 「as of」 date defaults to today; change it to see your week on any other day.
- Read your results: See the estimated due date, your current week and day, and your trimester.
- Check the milestones: The list shows when each trimester ends, week 24 (viability), week 39 (full term), and week 40 (due date) — milestones already passed are marked with a check.
Features
- Naegele’s rule: due date = LMP + 280 days (40 weeks)
- Current pregnancy week shown as weeks + days (e.g. 「12 weeks 3 days」)
- Trimester indicator (first, second, or third)
- Progress bar showing how far along the pregnancy is
- Milestone timeline: end of 1st/2nd trimester, viability at week 24, full term at week 39, due date at week 40
- 100% client-side — no data is uploaded or stored
Understanding Naegele’s Rule
Naegele’s rule dates back to the 19th century and remains the most common way to estimate a due date. Count 280 days — exactly 40 weeks — from the first day of the last menstrual period. The method assumes conception happened about two weeks after that date, matching a textbook 28-day cycle. Because real cycles vary, the estimate carries a natural margin: a normal delivery anywhere from week 37 to week 42 is considered on time. Early ultrasound, which measures the embryo directly, is the most reliable way to confirm or adjust the date — this tool gives you a solid first estimate to bring to that appointment.
Use Cases
- Early planning: Get a first due-date estimate right after a positive test, before your first prenatal visit.
- Tracking progress: Check which week and trimester you are in on any given date.
- Scheduling: See when the second trimester starts or when full term begins to plan leave, travel, or appointments.
- Sharing: Give family a concrete date and milestone timeline.
A Note on Accuracy
Only a small share of babies arrive exactly on the estimated date, so treat it as the center of a window rather than a fixed appointment. If your cycle differs from 28 days, or if your ultrasound dating disagrees, your healthcare provider’s estimate takes precedence. This calculator is for orientation and planning — it does not replace medical advice, and any concerns about your pregnancy belong with your doctor or midwife.
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Frequently Asked Questions
How is the due date calculated?
This calculator uses Naegele's rule, the standard method used by most healthcare providers: your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period (LMP). It assumes a regular 28-day cycle with ovulation on day 14.
How accurate is the estimated due date?
It is an estimate, not a guarantee. Only about 4-5% of babies are born exactly on their due date; most arrive within a window of roughly two weeks before or after. Ultrasound measurements in early pregnancy can refine the date.
What if my cycle is longer or shorter than 28 days?
Naegele's rule assumes a 28-day cycle. If your cycle is regularly longer, your actual due date is likely a bit later; if shorter, a bit earlier. Your doctor or midwife can adjust the date accordingly, often with an early ultrasound.
What do the milestones mean?
The milestone list shows the end of the first trimester (week 13), the week of viability (week 24, when survival outside the womb becomes possible), the end of the second trimester (week 27), full term (week 39), and the estimated due date (week 40).
Is this tool a substitute for medical advice?
No. This calculator provides an estimate for informational purposes only. Always consult your doctor or midwife for confirmation of your due date and for any questions about your pregnancy.
Is my data stored or uploaded anywhere?
No. All calculations run entirely in your browser. The date you enter never leaves your device.