Due Date Calculator by Last Period (LMP)
Enter the first day of your last period and get your estimated due date with Naegele’s rule.
Your LMP due date, computed privately in your browserEstimates only — not medical advice. Confirm all dates and milestones with your doctor or midwife. Read our full disclaimer.
How this due date is calculated
Enter the first day of your last menstrual period (LMP). Your due date is that date plus 280 days (40 weeks), using Naegele’s rule, adjusted for your cycle length if it differs from 28 days.
Naegele’s rule dates from 1812, when German obstetrician Franz Naegele observed that pregnancies anchored to a known last period averaged 280 days. It still works because human gestation is remarkably consistent once you fix that anchor. The only real assumption is a 28-day cycle with ovulation near day 14 — which is exactly why this site separates cycle-length and luteal-phase adjustments onto their own pages instead of baking one average into every result.
What to expect
Your "last period" means the first day of your most recent menstrual period — the one before you became pregnant. Pregnancy is counted from this date, not from conception, which is why a due date based on LMP is about two weeks longer than the time since conception.
Naegele's rule is the classic method: add 280 days (40 weeks) to the first day of your LMP. This assumes a 28-day cycle with ovulation around day 14. If your cycles are regular but longer or shorter than 28 days, the calculator above can adjust using your cycle length, which shifts the due date by the difference from 28 days.
Example: if your LMP was January 1, adding 280 days lands on about October 8 of the same year. That is the estimated due date — the midpoint of a window in which most babies arrive.
LMP-based dating is a good starting point, but it is less precise if your cycles are irregular or you are not sure of the date. An early ultrasound (usually between 8 and 13 weeks) measures the baby and gives the most accurate due date, which your provider will usually adopt if it differs from the LMP estimate by more than a few days.
Many people find it helpful to also track how far along they are week by week. Once you have a due date, the "How far along am I" tool shows your current week, trimester, and days remaining.
Important note
These calculators use standard formulas — Naegele's rule (LMP + 280 days) and common IVF conventions (transfer date + 260–263 days). They give estimates for planning only. They are not a substitute for prenatal care, and only your provider can confirm your due date and track your baby's development. See the disclaimer for details.
FAQ
Can I use ovulation date instead of LMP?
Yes — ovulation is usually about two weeks after the first day of your LMP, and an ovulation-based date is often more accurate if you tracked it. Switch to the By conception calculator and enter your ovulation or conception date.
How do I calculate my due date by ovulation date?
Ovulation is effectively the conception date. Enter your ovulation date in the By conception calculator; your due date is 266 days (38 weeks) later.