What this calculator does

This due date calculator estimates the expected due date from the first day of the last menstrual period. It is a guide for discussion with a healthcare professional.

Formula used

The standard estimate adds 280 days to the first day of the last menstrual period, with a simple adjustment for cycles longer or shorter than 28 days.

How to read the result

Due dates are estimates. Early ultrasound and clinical context can change dating, especially with irregular cycles or assisted reproduction.

Assumptions

  • Assumes the last menstrual period date is known.
  • Uses a 28-day cycle as the baseline.
  • Not a substitute for maternity care.

Sources and checks

Source links are included so you can check important formulas, thresholds and assumptions.

Frequently asked questions

Is the due date exact?

No. It is an estimate used to track pregnancy progress. Many babies are born before or after the estimated date.