Due Date Calculator (Naegele)

You want the due date without waiting for the next appointment or spinning a pregnancy wheel. Enter the LMP, the first day of the last menstrual period, and the tool returns the estimated due date. Under the hood it adds 280 days, that is 40 weeks, to the date you typed. The output uses year-month-day order, so 2026-05-08 means the eighth of May 2026.

The Naegele rule people memorise is add a year, subtract three months, add seven days. It aims at the same target, but since months run 28 to 31 days, that calendar shortcut drifts a day or two from the straight count. For an LMP of 10 March 2025 the shortcut lands on 17 December while adding 280 days lands on 15 December. This tool uses the 280 days, which is what 40 weeks actually means.

Treat the due date as a target, not a booking. Term birth spans 37 to 42 weeks, and arriving on the exact date is the exception rather than the rule. Naegele also assumes a 28-day cycle with ovulation halfway through, so a longer cycle usually pushes the date later. The first-trimester ultrasound has the final word and redates the pregnancy when it disagrees with the LMP. The arithmetic runs in your browser.