Pregnancy Due Date Calculator

Pregnancy due date calculator from first day of last period — get your estimated delivery date. Use the free Pregnancy Due Date Calculator below — enter your values and get instant results, right in your browser. Naegele rule: estimated due date = first day of last menstrual period + 280 days (40 weeks). Equivalent shortcut: LMP − 3 months + 7 days + 1 year. Assumes a regular 28-day cycle.

How to use the Pregnancy Due Date Calculator

One of the first questions after a positive test is: when is the baby due? Doctors estimate it from the first day of the last menstrual period using Naegele rule — add 280 days (40 weeks). This pregnancy due date calculator from first day of last period does that math instantly and privately in your browser, with no account and no data sent anywhere. Remember it is an estimate: only about 5% of babies arrive exactly on their due date.

  1. Open the Add/Subtract tab of the calculator.
  2. Enter the first day of your last menstrual period as the start date.
  3. Choose the + operator, type 280 and select days.
  4. Click Calculate to see your estimated due date and its weekday.
  5. Share the date with your doctor, who will confirm it with an ultrasound.

Formula

Naegele rule: estimated due date = first day of last menstrual period + 280 days (40 weeks). Equivalent shortcut: LMP − 3 months + 7 days + 1 year. Assumes a regular 28-day cycle.

Worked example

Last period started 1 January 2026. Add 280 days and click Calculate. Estimated due date: 8 October 2026, a Thursday. Your doctor will refine this with a first-trimester ultrasound scan. Only about five percent of babies arrive on the exact date, so treat it as the center of a two-week window.

Frequently asked questions

It is an estimate — only around 5% of babies are born on the exact date. Most arrive within two weeks before or after.

Naegele rule assumes a 28-day cycle. With longer or shorter cycles, shift the estimate by the difference and confirm with your doctor.

Yes. The calculation runs entirely in your browser and nothing is uploaded or stored anywhere.

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