Due Date Calculator
Calculate Due Date by entering start and end dates or times. Get precise durations in years, months, days, hours, and minutes.
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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.
Last reviewed: January 2026
Worked Examples
Example 1: Due Date from Last Menstrual Period
Example 2: Due Date from Known Conception Date
Background & Theory
The Due Date Calculator — Pregnancy Due Date Estimator applies the following established principles and formulas. Date and time calculations underpin a vast range of applications from financial settlement to scheduling and age verification. The complexity arises because civil timekeeping uses irregular units: months have 28, 29, 30, or 31 days; years have 365 or 366 days; hours, minutes, and seconds use base-60 arithmetic; and time zones introduce offsets ranging from -12:00 to +14:00 relative to UTC. The Gregorian calendar's leap year rule is a compound condition: a year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. Thus 1900 was not a leap year but 2000 was. This rule keeps the calendar synchronized with the solar year to within about 26 seconds per year. For algorithmic date calculations, the Julian Day Number provides a continuous integer count of days since January 1, 4713 BCE, eliminating the irregularity of calendar months and making interval arithmetic straightforward. The Unix epoch, by contrast, counts seconds since 00:00:00 UTC on January 1, 1970, and is the basis of POSIX time used in most computing systems. ISO 8601 standardizes date and time representation as YYYY-MM-DD and combined datetime as YYYY-MM-DDTHH:MM:SS±HH:MM, ensuring unambiguous machine-readable interchange across locales that would otherwise differ in day/month/year ordering. Business day calculation requires excluding weekends and, optionally, a jurisdiction-specific list of public holidays. Duration calculations expressed in years, months, and days must account for the variable length of months, making them non-commutative: the interval from January 31 to February 28 is different from the interval from February 28 to March 31. Age calculation algorithms must handle the edge case of birthdays on February 29 and ensure that a person born on December 31 is not counted as one year older on January 1 of the following year until the clock passes midnight. Zeller's Congruence provides a closed-form formula to determine the day of the week for any Gregorian or Julian calendar date using only integer arithmetic.
History
The history behind the Due Date Calculator — Pregnancy Due Date Estimator traces back through the following developments. The need to track time and predict astronomical events gave rise to calendrical systems independently across many civilizations. The Babylonians, around 2000 BCE, developed a lunisolar calendar with 12 months of alternating 29 and 30 days, inserting an intercalary month periodically to keep pace with the solar year. They also divided the day into 24 hours and the hour into 60 minutes, a sexagesimal convention that persists in every modern clock. The Egyptian civil calendar used 12 months of exactly 30 days plus five epagomenal days, totaling 365 days. Though simple for administrative purposes, it drifted against the solar year by one day every four years. Julius Caesar, advised by the Egyptian astronomer Sosigenes, reformed the Roman calendar in 45 BCE. The Julian calendar introduced a 365-day year with a leap day every four years, a system that served Europe for over sixteen centuries. By the 16th century, the accumulated error of the Julian calendar had shifted the spring equinox ten days from its ecclesiastically mandated date, disrupting the calculation of Easter. Pope Gregory XIII commissioned the calendar reform that bears his name, and the Gregorian calendar was introduced in Catholic countries in October 1582. The transition required skipping ten days: October 4 was followed by October 15. Protestant and Orthodox countries adopted the reform slowly; Britain and its colonies switched in 1752, Russia not until 1918, and Greece in 1923. The expansion of railways in the 1840s created an urgent practical problem: each city operated on its own local solar time, making train timetables impossible to coordinate. British railways adopted Greenwich Mean Time as a standard in 1847. The International Meridian Conference of 1884 in Washington formalized the prime meridian at Greenwich and established the global framework of 24 time zones. Daylight saving time was first adopted nationally during World War I to reduce coal consumption. The development of atomic clocks after World War II led to the definition of Coordinated Universal Time (UTC) in 1960, accurate to nanoseconds. The Y2K problem of 1999-2000 demonstrated that two-digit year storage in legacy systems could cause widespread failures, prompting a global remediation effort costing an estimated 300 to 600 billion dollars.
Key Features
- Calculate the exact difference between any two dates expressed in days, weeks, months, and years simultaneously, accounting for leap years and varying month lengths.
- Add or subtract any combination of years, months, weeks, and days from a starting date to determine a precise future or past date, with results shown in a full calendar format.
- Compute a person's exact age from their birthdate in years, months, and days as of today or any specified reference date, suitable for legal, medical, and personal use.
- Count business days between two dates by excluding weekends and optionally filtering out public holidays from a configurable set of regional holiday calendars.
- Display a live countdown to any target date and time showing the remaining years, months, days, hours, minutes, and seconds, updating in real time.
- Convert a specific date and time between any two IANA time zones, correctly handling daylight saving time transitions and historical offset changes.
- Determine the day of the week for any historical or future date using the proleptic Gregorian calendar, supporting dates ranging from antiquity through far-future years.
- Format a calculated duration in ISO 8601 interval notation as well as plain human-readable text such as '2 years, 4 months, and 11 days' for use in documentation and APIs.
Frequently Asked Questions
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?
Yes, leap years add February 29, extending the year to 366 days and affecting any date range that spans that date. A period from January 1 to December 31 covers 365 days in a regular year but 366 in a leap year. Similarly, 'one year from February 28' in a non-leap year is February 28, but in a leap year the next day (February 29) also exists, so applications must define whether 'one year later' maps to February 28 or February 29. Financial instruments like bonds and loans use specific day-count conventions (Actual/360, Actual/365, Actual/Actual) to handle these edge cases consistently.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
What inputs do I need to use Due Date Calculator accurately?
Each field is labelled with the required unit (metric or imperial). Gather your source values before starting — for example, a weight measurement in kilograms, a distance in metres, or a dollar amount — and enter them exactly as measured. The formula section on this page lists every variable and explains what each represents.
How do I verify Due Date Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.