Due Date Estimator
Estimate a baby's due date and conception date from the first day of the last period.
About Due Date Estimator
A typical pregnancy lasts about 280 days, or 40 weeks, counted from the first day of the last menstrual period. The due date is that span added to the LMP, while conception is estimated about two weeks later when ovulation usually happens. Knowing the approximate due date sets the timeline for prenatal visits, ultrasound scans and maternity leave planning, so it is one of the first numbers an expectant family wants.
Formula
Due Date
Due date = first day of last period + pregnancy length (usually 280 days); conception ≈ LMP + 14 days
- LMP
- First day of the last menstrual period
- gestation
- Length of pregnancy in days
Worked example
With the default values below, Due Date Estimator returns Due Date of January 15, 2026. Change any input above and every figure here is recalculated in your browser.
Inputs used
- LMP year
- 2,025
- LMP monthaccepts 1–12
- 4
- LMP dayaccepts 1–31
- 10
- Pregnancy length (days)accepts 150–320
- 280
Results
- Due Date
- January 15, 2026
- Due Date (day of year)
- 15
- Estimated Conception
- April 24, 2025
Step by step
- 1Values used
- 2Due Date
- 3Due Date= January 15, 2026
- 4Due Date (day of year)= 15
- 5Estimated Conception= April 24, 2025
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- Due dates are estimates; only about four percent of babies arrive exactly on the calculated date.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Due dates are estimates; only about four percent of babies arrive exactly on the calculated date.
Frequently Asked Questions
How is Due Date calculated?
Due date = first day of last period + pregnancy length (usually 280 days); conception ≈ LMP + 14 days A typical pregnancy lasts about 280 days, or 40 weeks, counted from the first day of the last menstrual period. The due date is that span added to the LMP, while conception is estimated about two weeks later when ovulation usually happens.
Why does Due Date matter?
Knowing the approximate due date sets the timeline for prenatal visits, ultrasound scans and maternity leave planning, so it is one of the first numbers an expectant family wants.
What values do I need to enter?
This calculator takes 4 inputs: LMP year, LMP month, LMP day, Pregnancy length (days). The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.