ESTIMATE A DUE DATE
Calculator inputs and results
Calculate an estimated due date from the first day of the last menstrual period, conception date or known due date. Results follow standard dating conventions and should be confirmed with a healthcare professional.
How to use this calculator
What the result includes
- Naegele's rule with cycle adjustment
- Week + trimester from any input mode
- Fruit-sized journey bar (wk 4–40)
- Trimester overview cards
- Informational-only disclaimers
How due dates are estimated 🧮
The classic formula is Naegele's rule (19th century, still in everyday use): take the first day of the last menstrual period (LMP), add 7 days, subtract 3 months — equivalently, LMP + 280 days (40 weeks). If your cycle isn't 28 days, providers adjust (this tool adds the difference). From a known conception date, pregnancy is dated at 266 days (38 weeks). Early ultrasound dating is more precise than either, which is exactly why you should confirm with your provider.
Worked example
| Input | Math | Due date |
|---|---|---|
| LMP: Jan 5, 2026 (28-day cycle) | +280 days | October 12, 2026 |
| LMP: Jan 5, 2026 (30-day cycle) | +282 days | October 14, 2026 |
| Conception: Jan 19, 2026 | +266 days | October 12, 2026 |
How to interpret the estimate
An estimated due date is a calendar reference, not a prediction of the exact day labor will begin. The calculation assumes a typical relationship between the last menstrual period and ovulation; irregular cycles, uncertain dates and assisted conception can change which starting point is most useful.
- Use one documented date: once your care team establishes an estimated due date, use that date for appointments and planning.
- Early ultrasound is the clinical reference: ACOG describes first-trimester ultrasound as the most accurate way to establish or confirm gestational age.
- Later changes should be uncommon: ACOG says subsequent changes to an established due date should be reserved for rare circumstances and documented.
- The calculator does not assess pregnancy health: symptoms, measurements and clinical decisions require a healthcare professional.
The fruit scale, decoded 🍓
The fruit comparisons are informal visual analogies. They vary by publisher and do not represent a clinical measurement; healthcare professionals use appropriate examination and imaging data when assessing a pregnancy.
After arrival, the Baby Age Calculator takes over (months, weeks, corrected age). For the month-by-month guide, see Your Baby's First Year. 🍼
Sources & limits
Due-date calculations are estimates. Clinical dating and prenatal decisions belong with a healthcare professional.
Sources accessed: August 8, 2026.