Due Date Calculator
Calculate Due Date by entering start and end dates or times. Get precise durations in years, months, days, hours, and minutes.
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy
Due Date Calculator Formula
Due Date = LMP + 280 days (Naegele's Rule)
Naegele's rule calculates the estimated due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. From a known conception date, add 266 days (38 weeks). Only about 5% of babies are born on their exact due date — most arrive within a 2-week window.
Due Date Calculator — Worked Examples
Example 1: Due Date from Last Menstrual Period
Problem:Your last menstrual period started on January 15, 2025. Calculate your due date and current pregnancy status.
Solution:Using Naegele's rule:\nLMP = January 15, 2025\nDue date = LMP + 280 days = October 22, 2025\n\nEstimated conception = LMP + 14 days = January 29, 2025\nFirst trimester ends: April 9, 2025 (week 12)\nSecond trimester ends: July 16, 2025 (week 26)\nFull term begins: October 1, 2025 (week 37)
Result:Due Date: October 22, 2025 | Conception ~January 29, 2025
Example 2: Due Date from Known Conception Date
Problem:Conception occurred through IVF on March 1, 2025. When is the due date?
Solution:Known conception date: March 1, 2025\nDue date = Conception + 266 days = November 22, 2025\n\nEquivalent LMP = March 1 - 14 days = February 15, 2025\nFirst trimester ends: May 17, 2025\nViability milestone (24 weeks): August 2, 2025\nFull term (39 weeks): November 15, 2025
Result:Due Date: November 22, 2025 | LMP equivalent: February 15, 2025
Due Date Calculator — Frequently Asked Questions
How is the due date calculated?
The due date is calculated using Naegele's rule, the standard method used by healthcare providers worldwide. Starting from the first day of your last menstrual period (LMP), the formula adds 280 days (40 weeks) to estimate the due date. This assumes a 28-day menstrual cycle with ovulation occurring on day 14. If calculating from a known conception date, 266 days (38 weeks) are added instead, since conception typically occurs about 14 days after the LMP. The due date is an estimate — only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the due date (between 38-42 weeks). Your healthcare provider may adjust the due date based on early ultrasound measurements, which can be more accurate than LMP-based calculations, especially if your cycle is irregular.
How accurate is the due date estimation?
Due date estimates have inherent uncertainty regardless of the method used. LMP-based calculations assume a regular 28-day cycle with ovulation on day 14, but many women have irregular cycles or ovulate on different days. Studies show that only about 5% of babies are born on their exact due date, while approximately 80% are born within 2 weeks of the estimated date (between 38 and 42 weeks). First-trimester ultrasound (before 14 weeks) is the most accurate dating method, with accuracy of plus or minus 5-7 days. Second-trimester ultrasound has accuracy of plus or minus 1-2 weeks, and third-trimester dating is accurate to plus or minus 2-3 weeks. First-time mothers tend to deliver slightly later than their due date, while subsequent pregnancies may deliver earlier. Your provider may adjust the due date if ultrasound measurements differ significantly from LMP calculations.
Does a leap year affect date difference calculations?
Leap years add February 29, extending the year to 366 days and affecting any date range spanning that date. January 1 to December 31 is 365 days in a regular year but 366 in a leap year. Applications must define whether "one year from February 28" maps to February 28 or 29 in a leap year. Financial instruments use specific day-count conventions — Actual/360, Actual/365, or Actual/Actual — to handle these edge cases consistently.