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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.
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What does this calculator estimate?
The pregnancy calculator estimates a due date and gestational timeline from a last menstrual period, conception date, or a known due date, and shows the current week and trimester.
The Naegele rule explained
The most common way to estimate a due date is Naegele's rule: take the first day of the last menstrual period and add 280 days (40 weeks), adjusted by subtracting the difference between a 28-day cycle and the actual cycle length. It assumes ovulation around day 14 and a 266-day average from conception, which is why conception-based estimates differ.
How gestational age is counted
Gestational age counts from the first day of the last menstrual period, which is about two weeks before conception. That is why a pregnancy at 10 weeks gestational age is only about 8 weeks from conception. Obstetricians use weeks and days (for example, 38 weeks 2 days) to track progress.
Why the due date is an estimate
Only about 4–5% of babies are born on their exact due date. Cycles vary, ovulation timing varies, and the length of pregnancy varies. Ultrasound measurements in the first trimester are often the most accurate way to date a pregnancy, so always confirm your dates with a provider.
How this calculator works
Formula
Using the last menstrual period (LMP), the estimated due date is LMP + 280 days adjusted for cycle length (standard Naegele rule assumes a 28-day cycle, so a 30-day cycle adds 2 days). Conception is estimated as LMP + (cycle − 14) days, and gestational age counts from LMP. From a known due date or conception date the other dates are back-calculated.
Worked example
With a last menstrual period of November 20, 2025 and a 28-day cycle, the estimated due date is August 27, 2026, conception is estimated around December 4, 2025, and by August 15, 2026 the pregnancy is about 38 weeks and 2 days along (96% complete, ~12 days left).
Assumptions to verify
- The standard Naegele rule (280 days from LMP, 266 days from conception) is used, adjusted for the entered cycle length.
- The entered dates are estimates; actual timing varies and is best confirmed by a healthcare provider and ultrasound.
- Cycle length is assumed between 22 and 44 days.
- This tool provides general estimates for planning and education only; it is not medical advice and not a substitute for a qualified healthcare professional.
Frequently asked questions
How is a due date calculated?
The most common method is Naegele's rule: first day of the last menstrual period + 280 days, adjusted for cycle length. From a conception date, add 266 days. This calculator supports both, plus back-calculating from a known due date.
Why is my cycle length used?
A longer cycle shifts ovulation (and thus conception and the due date) later, while a shorter cycle shifts it earlier. Adjusting for your average cycle makes the estimate more accurate than assuming a flat 28 days.
What week am I in pregnancy?
Gestational age counts from the first day of your last menstrual period. This calculator shows the current weeks and days along, plus which trimester you are in.
How accurate is an estimated due date?
Only a small percentage of babies arrive exactly on the due date, and a normal pregnancy can last from 37 to 42 weeks. The date is an estimate; early ultrasound dating is generally the most reliable.
What is the difference between LMP and conception dating?
LMP dating counts from the first day of your last period (about two weeks before conception), while conception dating counts from the estimated day of conception. The two approaches give the same due date when a standard 28-day cycle and day-14 ovulation are assumed.
Cite this tool
BoringToolsKit. “Pregnancy Calculator.” boringtoolskit.com/pregnancy-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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