Due Date Calculator
Adjust the inputs below. Results update as you type.
How it works
Naegele's rule: LMP + 280 days (40 weeks). Ultrasound dating in the first trimester (crown-rump length) is most accurate—within ±5 days at 8–12 weeks. Second-trimester scans have ±2-week error. Only 5% of babies arrive on their exact due date; most arrive within 2 weeks. Due dates are estimates; discuss delivery timing with your OB or midwife.
Input guidance
- Use measured values (recent weight, height, activity level) rather than guesses.
- Recalculate every few weeks as your routine and body metrics change.
- Pair screening metrics with context such as sleep, stress, and medical history.
The formula
Naegele's rule: due date = first day of last menstrual period + 1 year − 3 months + 7 days, equivalent to LMP + 280 days.
Worked example
An LMP of March 10 gives a due date of about December 17 (add a year, subtract three months, add seven days).
More examples to test
- Maintenance case: keep activity constant and observe expected baseline ranges.
- Adjustment case: change only one major variable to understand sensitivity.
How to interpret results
Use results for awareness and planning, then validate with qualified medical or nutrition professionals for individualized decisions.
When this can be inaccurate
Population formulas cannot account for all biological factors such as medical conditions, medication effects, body composition differences, and genetics.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
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General wellness resource.
Frequently asked questions
How is the due date calculated?+
Most commonly with Naegele's rule from the last menstrual period, assuming a regular 28-day cycle and ovulation around day 14.
Can an ultrasound change it?+
Yes. Early ultrasound dating is often more accurate than LMP, especially with irregular cycles, and providers may adjust the due date.
Is a due date a deadline?+
No. It marks 40 weeks, but a full-term birth is anywhere from 37 to 42 weeks, so arrival around the date is normal.
Can this calculator diagnose a condition?+
No. It is a screening and education tool only, not a clinical diagnosis.
When should I seek professional advice?+
Seek professional advice when results are extreme, persistent, or conflict with symptoms, goals, or prior medical guidance.