Dates and durations · Calendar
Estimate a due date from the last period
One date: the first day of the last period. The tool adds 280 days for the due date and 14 days for a model conception.
Calculate
How the calculator works
Naegele’s rule counts 280 days (40 weeks) from the first day of the last period, not from conception. Conception is shown at day 14, assuming a 28-day cycle with ovulation on day 14. A longer cycle shifts everything; a first-trimester ultrasound often redates. Civil UTC dates, no clock time.
Formula and method
due = LMP + 280 days
40 weeks of amenorrhea, a common obstetric convention.
conception ≈ LMP + 14 days
Day-14 ovulation on a 28-day cycle.
Naegele’s 19th-century shortcut (add a year, subtract three months, add seven days) equals +280 days on a non-leap calendar. We add 280 civil days, including 29 February. This is not a diagnosis and not the day birth “must” happen; most births spread around the due date.
Worked example
LMP on 1 January 2026
lmp = 2026-01-01.
- + 14 days → conception 15 January 2026.
- + 280 days → due 8 October 2026.
Estimated due date: 8 October 2026.
Input notes
- First day of last period
- First day of the last period. If cycles are irregular, ultrasound dating is more reliable than this rule.
Assumptions and limits
Assumptions
- 28-day cycle, ovulation on day 14.
- Singleton pregnancy dated from LMP.
- 280 civil days, including leap days.
Limits
- No crown-rump measurement.
- No correction for a 35-day cycle.
- Not medical advice.
How to read the result
The due date is a median, not a deadline. Birth two weeks either side is common. If an ultrasound already gave an EDD, that is what prenatal care uses. A clinician dates the pregnancy when there is doubt.
Common mistakes
Entering the conception day.
The field wants the first day of the period, 14 days earlier in this model.
Adding nine calendar months.
Nine months are not always 280 days. The tool counts days.
Treating the due date as a guaranteed birthday.
It is 40 weeks of amenorrhea, not an appointment.
Comparison
| Event | Offset | If LMP = 1 Jan 2026 |
|---|---|---|
| Conception (model) | + 14 d | 15 Jan 2026 |
| 12 weeks of amenorrhea | + 84 d | 26 Mar 2026 |
| Due date (40w) | + 280 d | 8 Oct 2026 |
Related calculations
- Age calculatorThe age shown is civil: completed years, then months, then leftover days. It is not the number of days divided by 365.25. An empty “Age on” field means today.
- Date differenceThe main result is a decomposition: 2 years, 5 months, and 10 days, for example. Total days sit beside it. That is not the same need as “how many stay nights.”
- Ovulation calculatorOne date: the first day of the last period. The tool places ovulation on day 14 and a fertile window from day 10 to day 17.
- Pregnancy weekThe LMP and, if you want, a date. The tool counts weeks of amenorrhea and shows the trimester.
Frequently asked questions
How is a due date calculated?
Often LMP + 280 days.
That is Naegele’s rule in weeks of amenorrhea. An early ultrasound can shift the date if LMP is uncertain or cycles are irregular.
Why 40 weeks if conception is at week 2?
Counting starts from the period, not fertilization.
Forty weeks of amenorrhea include the two weeks before ovulation. Embryonic life is closer to 38 weeks.
What if my cycle is 35 days?
This page does not adjust for cycle length.
A longer cycle usually delays ovulation. Clinicians then rely on ultrasound. The model here stays 28 days / day 14.
Author and update
Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.
This calculation uses a standard mathematical identity. See also the methodology.
Count the days to the due date?
Days between two dates gives the remaining civil days.
Days between datesA calendar tool, not a medical device. It does not date a pregnancy and does not replace prenatal care.
Category: Dates and durations