Due Date Calculator

When to use it

An estimated due date is mostly a planning tool: it tells you which month to clear at work, when to have the bag packed, and how the appointments will fall. This page applies the standard rule from the first day of the last period, adjusted for your usual cycle length.

It is an estimate and nothing more. Dating is confirmed by a midwife or doctor, often with a scan, and the estimate can move by a week or more. Treat the date as the centre of a window, not as an appointment.

How the maths works

Naegele's rule adds 280 days — forty weeks — to the first day of the last menstrual period. The tool then adjusts for cycle length: one extra day for every day the cycle exceeds 28. A 32-day cycle therefore adds four days, and a 24-day cycle takes four away. The cycle field accepts 20 to 45 days.

The detail line also reports how many weeks and days have passed since that first day, which is the figure most appointments quote. If the date you enter is in the future, the page says so instead of returning a negative count.

A worked example

First day of the last period: 1 January 2026, with a 28-day cycle.

1. Add 280 days to 1 January 2026.
2. January has 30 days left after the 1st, February 28, March 31, April 30, May 31, June 30, July 31, August 31 and September 30 — 272 days by 30 September.
3. Eight days remain: 8 October 2026.
4. With a 32-day cycle, add four more days: 12 October 2026.

Cycle adjustments

A week of difference sounds small until it lands on a work deadline or a family visit, which is why the cycle field exists at all.

Common mistakes

• Counting from conception. The rule starts about two weeks earlier, at the first day of the last period, so a conception date gives a date roughly a fortnight late.
• Guessing the cycle. If your cycles vary, use the average of the last few rather than a single unusually long or short one.
• Treating the date as fixed. Only a minority of births land on the estimated day, and a scan often revises it.
• Entering a date in the future. The page flags it, because the count of weeks would be meaningless.

Keep exploring

• Week Number Calculator — turn any date into its ISO week number.
• Date Difference — the exact gap between two dates.
• Add Days to a Date — move a date forward by a set number of days.
• Guide: Working with Dates and Time.