Calculators
How to Calculate a Pregnancy Due Date
AllinOne Editorial · Published 2026-09-16
Naegele’s rule is a 40-week calendar from LMP. Ultrasound and cycle length can move the date. This page will not replace a clinician.
The common due-date estimate is Naegele’s rule: 280 days (40 weeks) after the first day of the last menstrual period (LMP). Conception is often estimated 14 days after that LMP, which assumes a 28-day cycle.
Formulas this calculator uses
- Due date = LMP + 280 days.
- Conception estimate = LMP + 14 days.
- Gestational age is whole days from LMP to the “as of” date, shown as weeks + leftover days.
- Trimester 1 is days 0–90, trimester 2 is 91–188, trimester 3 is 189–280.
Worked example
LMP on 15 January 2026 gives a due date of 22 October 2026 and a conception estimate of 29 January 2026. On 12 September 2026 that pregnancy is 34 weeks and 2 days.
You can also start from a known due date or a conception date. The page is a calendar, not a diagnosis.
Frequently asked questions
Is this medical advice?+
No. Ultrasound dating, cycle length and clinical judgment can move the due date by days or weeks.
What if my cycle is not 28 days?+
The +14-day conception estimate assumes ovulation about two weeks after LMP. Use the conception mode if you have a better estimate.
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