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Calculation Machine

Due Date Calculator

Estimated delivery date and pregnancy milestones.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Estimated due date
2026-12-06 (Sunday)
Gestational age today
21 weeks, 2 days
Trimester
Second
Conception (approx)
2026-03-15
Days remaining
131

Quick answer

Naegele's rule estimates the due date as 280 days from the first day of the last menstrual period.

The formula

Due date = LMP + 280 days

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • First day of last period2026-03-01 YYYY-MM-DD
  • Cycle length28 days
  • Today's date2026-07-28 YYYY-MM-DD

Result

  • Estimated due date2026-12-06 (Sunday)
  • Gestational age today21 weeks, 2 days
  • TrimesterSecond
  • Conception (approx)2026-03-15
  • Days remaining131

How to use the Due Date Calculator

  1. 1Enter the first day of your last period.
  2. 2Adjust your average cycle length.
  3. 3Read the due date and current gestational age.

Frequently asked questions

How accurate is the due date?

Only about 4% of babies arrive on the estimated date; roughly 80% arrive within two weeks either side.

Why count from the last period?

Ovulation is hard to pin down, but the last period is a date most people know, so obstetrics standardised on it.

About this calculator

How a due date is estimated

Most due dates are calculated with Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, so if your cycles are longer or shorter, the date shifts by the difference.

Only around 4–5% of babies arrive on their exact due date. It is better thought of as the middle of a window — most babies are born between 38 and 42 weeks.

Example

If your last period started on 1 March and your cycles are 28 days, the estimated due date is 6 December. With a 32-day cycle, it moves four days later to 10 December.

When the date changes

  • A first-trimester ultrasound is more accurate than period dates, and your care team may adjust the date after it.
  • IVF due dates are calculated from the embryo transfer date instead.
  • Irregular cycles make period-based dates less reliable — mention this at your first appointment.

Sources

More questions

Can my due date change?

Yes. It is commonly revised after the dating scan, usually in the first trimester.

Are first babies born late?

On average first babies arrive slightly later, but the difference is only a few days.