Due Date Calculator
Enter the first day of your last menstrual period (LMP) and the tool estimates your due date using the classic 280-day / 40-week rule. It also shows the estimated conception date and your trimester boundaries. Educational estimate — your provider will confirm with ultrasound.
Last Menstrual Period
The 280-Day Rule
Pregnancy is counted from the first day of the LMP, not conception, because that date is easier to recall. Since ovulation usually happens ~14 days later, the fetus is about 266 days old at a 280-day "due date":
Due Date = LMP + 280 days (40 weeks) | Conception ≈ LMP + 14 days
Trimesters split the 40 weeks into three roughly equal stages used for monitoring: weeks 1–13, 14–27, and 28–40. An early ultrasound refines the date if your cycle differs from 28 days.
How Accurate Is It?
Only about 4% of babies arrive exactly on the due date. Most are born between 37 and 42 weeks, and the date is best read as a planning midpoint. Factors like irregular cycles, IVF (where conception is known exactly), and first-trimester ultrasound measurements all adjust the estimate your provider uses.
Frequently Asked Questions
How is a due date calculated?
LMP + 280 days (40 weeks). Assumes ovulation ~day 14; ~4% deliver exactly on it.
What if my cycle isn't 28 days?
The rule assumes day-14 ovulation. A first-trimester ultrasound gives a personalized date — tell your provider your cycle length.
What is the conception date estimate?
LMP + 14 days, near mid-cycle. The real fertile window is ~6 days, so it's approximate.
How accurate is the due date?
A 40-week estimate; most births fall between 37–42 weeks. It's a midpoint, not a precise prediction.