EDD Calculator (Estimated Due Date)
The EDD calculator estimates your due date by adding 280 days (adjusted for cycle length) to the first day of your last menstrual period.
verified_userReviewed by the Calculopedia editorial teamLast updated 2026-08-15
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quizExample
How this calculator works, with real numbers (no JavaScript needed):
Inputs
- First day of last menstrual period (LMP)
- 2026-01-15
- Average cycle length
- 28
Results
- Estimated due date
- 22 October 2026
- Due date
- October 22, 2026
- Weeks pregnant
- 32
- Current trimester
- Third trimester (weeks 28+)
functionsThe formula
The Estimated Due Date (EDD) is the predicted date when your baby will be born. This calculator adjusts for your actual cycle length, giving a more accurate estimate than the standard 280-day rule.
Naegele's rule (adjusted)
EDD = LMP + 280 days + (cycle length − 28)
If your LMP was 15 January 2026 and your cycle is 30 days:
EDD = 15 Jan + 280 + (30 − 28) = 15 Jan + 282 days
EDD = **14 October 2026**
Where the 280-day rule comes from
The underlying method is Naegele's rule, named after the German obstetrician Franz Karl Naegele, who described it in the early 1800s: take the first day of the last menstrual period, add one year, subtract three months and add seven days. That arithmetic works out to a pregnancy of 40 weeks, or 280 days, and it has been the backbone of due-date estimation ever since. The elegance of using the LMP is that it is a date most women can recall, even though conception actually happens about two weeks later.
Why cycle length matters
The standard 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, ovulation happens later, and the due date should be pushed forward accordingly.
| Cycle length | Adjustment |
|---|---|
| 26 days | −2 days |
| 28 days | 0 (standard) |
| 30 days | +2 days |
| 32 days | +4 days |
| 35 days | +7 days |
If your periods vary a lot from month to month, this adjustment becomes less reliable, because there is no single cycle length to anchor the calculation on. In that situation, an early ultrasound provides a far more dependable date.
Trimesters
| Trimester | Weeks |
|---|---|
| First | 1–13 |
| Second | 14–27 |
| Third | 28–40 |
Important note
Only about 4% of babies arrive exactly on their due date. Most are born within two weeks before or after. The due date is best understood as the centre of a wide window rather than a precise appointment. First-trimester ultrasound dating is the most accurate method. This calculator provides an estimate — always follow your doctor's guidance for medical decisions.
helpFrequently asked questions
question_markHow is EDD calculated?
Add 280 days to the first day of your last menstrual period. For a 28-day cycle, use standard Naegele's rule. For longer or shorter cycles, adjust by the difference from 28 days.
question_markWhat is EDD?
EDD stands for Estimated Due Date — the predicted date when your baby will be born, calculated from your last menstrual period and cycle length.
question_markHow accurate is the due date?
Only about 4% of babies are born on the exact due date. Most arrive within 2 weeks before or after. First-trimester ultrasound is the most accurate dating method.