Due date calculator using Naegele’s rule

Naegele’s rule estimates the due date from the first day of the last menstrual period: pregnancy is taken to last 280 days (40 weeks). The calculator shows the due date and the gestational age today or on a chosen reference date.

⚠︎ Learning tool for medical education – not for use in patients or for treatment decisions.

Calculator

–estimated due date

🔒 Calculated only in your browser – nothing is stored or transmitted.

How it is calculated

Estimated due date = first day of the last menstrual period + 280 days. Gestational age is counted from that day and given as completed weeks plus days: 12+3 means 12 completed weeks and 3 days. The classic calendar form of the rule (+ 7 days, − 3 months, + 1 year, i.e. 9 months and 7 days) may differ from the 280-day count by a few days depending on month lengths.

Limitations

The rule assumes a regular 28-day cycle with ovulation around day 14. Actual pregnancy length depends on cycle length, timing of ovulation, parity, ethnicity and body size, among others; the estimated date is therefore a guide, not a fixed date (Lawson 2021). Baskett and Nagele (2000) point out that Naegele’s original rule may have been misinterpreted, resulting in an earlier estimated date. ACOG (2017) names the last menstrual period and an early ultrasound examination as the basis for dating.

FAQ

How is the due date calculated with Naegele’s rule?▾

First day of the last menstrual period plus 280 days; in calendar form plus 7 days, minus 3 months, plus 1 year.

What does 12+3 weeks mean?▾

12 completed weeks and 3 days of gestation, counted from the first day of the last menstrual period.

Does the calculator account for different cycle lengths?▾

No. Adjusting for longer or shorter cycles is common, but we found no primary source with a defined formula, so the calculator uses the classic rule only.

References

  1. Lawson GW. Naegele’s rule and the length of pregnancy – a review. Aust N Z J Obstet Gynaecol 2021;61:177–82. · PubMed
  2. Baskett TF, Nagele F. Naegele’s rule: a reappraisal. BJOG 2000;107:1433–5. · PubMed
  3. ACOG Committee Opinion No. 700: Methods for estimating the due date. Obstet Gynecol 2017;129:e150–4. · PubMed

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Intended purpose: learning tool for medical education only. Not intended for use in patients or for diagnostic or treatment decisions; not a medical device. Calculations follow the cited original publications; no guarantee of completeness or correctness. Last updated: 10/2026 · Dr. Pascal Bafteh