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Pregnancy Due Date Calculator

Estimate your due date from the first day of your last period, see how many weeks pregnant you are, and when each stage of term begins.

Enter the first day of your last period to estimate your due date.

Estimate. A real-world figure based on the assumptions listed below, which you can change.

How this calculator works

The standard estimate counts 280 days, or 40 weeks, from the first day of your last menstrual period. That is the convention the American College of Obstetricians and Gynecologists (ACOG) uses. It counts from the period, not from conception, so the first two weeks of the count come before you were pregnant.

Naegele’s rule is the traditional hand method for the same thing: take the first day of your last period, add a year, subtract three months and add seven days. Because months are different lengths, it can land a day or two away from 280 days. The calculator shows both and tells you when they differ.

It also counts how far along you are, in completed weeks and days, and shows the dates of each stage of term as ACOG defines them: early term from 37 weeks, full term from 39, late term from 41 and postterm from 42.

This is an estimate, not a diagnosis. ACOG’s guidance is that an ultrasound early in pregnancy is the most accurate way to date it, and your doctor or midwife may set a different due date from it.

The formula

Exactly what happens to your numbers, step by step.

Due date (ACOG convention)
```
first day of last period + 280 days
```

Naegele’s rule

first day of last period + 1 year − 3 months + 7 days

How far along

days since the first day of the last period
= weeks × 7 + days

A worked example

A last period that began on March 15, 2026, counted on October 12, 2026.

What you enter

First day of last period: March 15, 2026
Count weeks as of: October 12, 2026

Step by step

Due date: March 15 + 280 days = Sunday, December 20, 2026
Naegele’s rule: December 22, 2026: two days later
On October 12: 30 weeks, 1 day pregnant; 69 days to go
Full term: December 13 to December 26, 2026

Due Sunday, December 20, 2026

The two methods differ because the nine months from March to December hold 275 days, so adding nine months and seven days comes to 282, not 280. ACOG uses the 280-day count.

Due dates, worked out

The 280-day rule applied to common questions.

1.My last period started on January 1, 2026. When is my due date?
1. Add 280 days (40 weeks) to January 1, 2026.
Thursday, October 8, 2026.
2.How many weeks pregnant am I on October 1, 2026, if my last period started July 1, 2026?
1. July 1 to October 1 is 92 days.
2. 92 days is 13 weeks and 1 day.
13 weeks, 1 day. The due date is Apr 7, 2027.
3.Why does Naegele’s rule give a different date for a period starting March 15?
1. 280 days after March 15, 2026 is Dec 20, 2026.
2. Naegele’s rule, nine months and seven days on, gives Dec 22, 2026.
They differ by 2 days because the nine months from March to December include several 31-day months. The 280-day count is the convention ACOG uses.
4.If my last period began on February 1, 2026, when am I full term?
1. Full term is 39 0/7 to 40 6/7 weeks: days 273 to 286 after the first day of the last period.
Nov 1, 2026 to Nov 14, 2026.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

The date entered is the first day of your last menstrual period.
Your cycle is around 28 days and you ovulated around the middle of it. That is what the 280-day convention assumes.
Weeks are counted in completed weeks and days, so 30 weeks 1 day means 30 full weeks and one more day.

What this cannot tell you

It cannot account for long, short or irregular cycles, or for a pregnancy dated by ultrasound or IVF. In those cases the due date your clinician gives you should take priority.
It is not medical advice. If you think you may be pregnant, or have any concern in pregnancy, talk to a doctor or midwife.

Questions people ask

How is a due date calculated?

By convention, as 280 days, or 40 weeks, after the first day of your last menstrual period. For a period that began on January 1, 2026, that is October 8, 2026. An early ultrasound can confirm or change it.

What is Naegele’s rule?

A shortcut for the same estimate: from the first day of your last period, add one year, subtract three months and add seven days. It usually matches the 280-day count exactly, but can be a day or two off because months have different numbers of days.

How many weeks pregnant am I?

Count the days from the first day of your last period to today and divide by seven: the whole number is the weeks, the remainder the days. Pregnancy is counted this way, from the period, which is why you are already considered two weeks along at conception.

What do early term, full term and late term mean?

They are ACOG’s definitions for a baby born at 37 weeks or later: early term is 37 weeks 0 days to 38 weeks 6 days, full term 39 weeks 0 days to 40 weeks 6 days, late term 41 weeks 0 days to 41 weeks 6 days, and postterm 42 weeks or more.

Is an ultrasound more accurate than the last period?

Yes. ACOG says an ultrasound in the first trimester is the most accurate way to establish how far along a pregnancy is. If it disagrees with the date from your last period by more than a set amount, your clinician will usually go by the ultrasound.

Sources and review

Sources

Methodology version 1.0.0 · Last reviewed October 12, 2026

Cite this calculator

For coursework, a resource list or your own site. How every calculator is built and checked is set out on the methodology page.

Citation

Chalk Calculator. (2026, October 12). Pregnancy Due Date Calculator (Methodology version 1.0.0). https://chalkcalculator.com/pregnancy-due-date-calculator/

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