How is the due date calculated?

It adds 280 days (40 weeks) to the first day of your last period, the basis of Naegele's rule, and then nudges the date by the difference between your cycle length and 28 days to account for earlier or later ovulation.

Why adjust for cycle length?

The 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation and conception shift accordingly, so the tool moves the due date by the same number of days.

How accurate is an estimated due date?

It is a best estimate, not a fixed appointment. Only about 4–5% of babies are born on their exact due date, and most arrive within a window of roughly two weeks on either side. An early ultrasound can refine the date.

When do the trimesters end?

This tool marks the first trimester ending around 12 weeks (84 days from the last period) and the second trimester ending around 26 weeks (182 days), with the third trimester running until birth.

Due Date Calculator

Last Period Date

Cycle Length28 days

Due Date--

Conception Date--

1st Trimester Ends--

2nd Trimester Ends--

A pregnancy due date calculator projects your estimated delivery date from the first day of your last menstrual period, adjusting for cycle length. Building on Naegele's rule of roughly 280 days of gestation, it also marks the likely conception date and the ends of the first and second trimesters so you can map out the pregnancy timeline. The result is an estimate; only about one in twenty babies arrives exactly on the due date.

Formula

Due date = last period date + 280 days + (cycle length − 28)

last period date: First day of your most recent menstrual period (LMP)
280: Average gestation in days (40 weeks) per Naegele's rule
cycle length − 28: Adjustment for cycles differing from the 28-day average

How it works

Enter the first day of your last menstrual period and your average cycle length in days.
The due date is set at 280 days after the last period, plus an adjustment of (cycle length − 28) days for cycles longer or shorter than the 28-day average.
It also estimates the conception date (about 14 days after the last period, plus the same adjustment) and fixes trimester boundaries at 84 days (end of first trimester) and 182 days (end of second).

Worked example

A last period beginning January 1 with a standard 28-day cycle.

Adjustment = 28 − 28 = 0 days.
Due date = January 1 + 280 days = October 8.
Conception ≈ January 1 + 14 = January 15; trimester ends at +84 days (March 26) and +182 days (July 2).

Estimated due date October 8, with first trimester ending March 26 and second ending July 2.

Frequently asked questions

How is the due date calculated?: It adds 280 days (40 weeks) to the first day of your last period, the basis of Naegele's rule, and then nudges the date by the difference between your cycle length and 28 days to account for earlier or later ovulation.
Why adjust for cycle length?: The 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation and conception shift accordingly, so the tool moves the due date by the same number of days.
How accurate is an estimated due date?: It is a best estimate, not a fixed appointment. Only about 4–5% of babies are born on their exact due date, and most arrive within a window of roughly two weeks on either side. An early ultrasound can refine the date.
When do the trimesters end?: This tool marks the first trimester ending around 12 weeks (84 days from the last period) and the second trimester ending around 26 weeks (182 days), with the third trimester running until birth.

Related calculators

Conception Calculator Pregnancy Calculator Ovulation Calculator