Due Date Calculator

How a due date is usually estimated

The standard due-date method starts from the first day of the last menstrual period, often shortened to LMP. By convention, pregnancy is dated as 280 days, or 40 weeks, from that date. This approach assumes a cycle of about 28 days with ovulation around day 14, which is why cycle length can matter when a due date calculator allows you to adjust it.

That is why a pregnancy due date calculator often asks for the first day of the last period rather than the day pregnancy was confirmed. In early pregnancy, clinicians use that estimated due date as a working reference for gestational age, scan timing, and general antenatal milestones.

Core due-date formulas

The calculator uses the same standard dating relationships used in many pregnancy tools. When the starting point is LMP, the estimated due date is counted forward by 280 days. When the starting point is conception, the estimate is usually counted forward by about 266 days, reflecting the typical gap between LMP-based gestational dating and conception itself.

Why the estimate can change

A due date is an estimate rather than a guarantee. Many people do not ovulate exactly on day 14, cycles are not always 28 days, and the first day of the last period may not be remembered exactly. This is why a due date based on dates alone can differ from a first-trimester ultrasound estimate.

In clinical care, early ultrasound is often used to confirm or revise the estimated due date because it can date a pregnancy more accurately than recall alone. That is especially important when the LMP is uncertain, cycles are irregular, or the scan measurement differs materially from the calendar estimate.

Using a due-date estimate safely

A due date calculator is useful for orientation, but it does not replace prenatal care. If the date of the last period is uncertain, if symptoms are unusual, or if there are concerns about bleeding, pain, or pregnancy timing, a clinician or midwife should review the pregnancy directly.

For most users, the most useful outputs are the estimated due date, estimated conception date, gestational age in weeks and days, and trimester. Those are planning figures, not medical diagnoses, and they should be confirmed with a clinician during routine antenatal care.