Due date calculator: estimate your pregnancy due date from the first day of your last period or your conception date. Free, private, runs in your browser.
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Due date calculator: estimate your pregnancy due date from the first day of your last period or your conception date. Free, private, runs in your browser.
280 days from LMP to due date
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
Formula
Due date = LMP + 280 days (40 weeks) or conception + 266 days.
BoringToolsKit. “Due Date Calculator.” boringtoolskit.com/due-date-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: LMP of January 6, 2026
Take a last menstrual period of January 6, 2026 in LMP mode. The calculator adds 280 days to that date. Counting forward: 25 days remain in January, then February contributes 28 (2026 is not a leap year), March 31, April 30, May 31, June 30, July 31, August 31, and September 30. That totals 267 days, reaching September 30. The remaining 13 days land on October 13, 2026, the estimated due date.
Conception is backed off 266 days from the due date, which equals 14 days after the LMP, so the tool reports January 20, 2026. This matches the textbook ovulation assumption of day 14 of a 28-day cycle. If you switch the mode to conception and enter January 20 instead, you get the same October 13 due date, because 266 + 14 = 280. Note the asymmetry: a cycle that runs 35 days shifts real ovulation to roughly day 21, which would move the true due date about a week later than this estimate.
The tool also flags the current trimester. Week 28, the start of the third trimester, begins 27 weeks (189 days) after the LMP, here July 14, 2026. The last 14 days of the count are week 40, running September 29 to October 13.
Where the 280-day assumption breaks down
Naegele's rule bakes in three assumptions: a 28-day cycle, ovulation on day 14, and a 40-week gestation from LMP. Real data cut against all three. Only about 4 to 5 percent of babies arrive on the estimated due date; the majority arrive in the 37 to 42 week window, and roughly half of first-time pregnancies pass the due date before labor starts.
Cycle length is the biggest lever. A woman with a 24-day cycle ovulates around day 10, so the baby is effectively 4 days further along than an LMP-based date suggests; a 35-day cycle shifts things about a week the other way. That is one reason guidelines recommend adjusting the due date whenever a first-trimester ultrasound disagrees with the LMP by more than 5 to 7 days. In practice, first-trimester crown-rump length dating changes the EDD for a substantial share of pregnancies, and the ultrasound date is treated as the authoritative one.
The 266-day conception offset is likewise an average. Actual implantation timing varies, which is why a conception date entered as a single specific day produces a false-precision estimate. If your cycle is irregular or longer than 32 days, treat the LMP result as a starting point and expect a first ultrasound to move it. The output is a planning aid for scheduling appointments and leave, not a delivery prediction, and it does not account for IVF transfer dates, which use an embryo-age offset of about 261 days for a day-5 blastocyst.
