# Pregnancy Due Date Calculator

URL: https://speedycalculator.com/calculators/health/due-date/
Last updated: 2026-10-25

> The due date calculator estimates your baby’s due date from the first day of your last menstrual period using Naegele’s rule (280 days), adjusted for your cycle length. It also shows how many weeks pregnant you are today.

## Inputs

- First day of last period
- Average cycle length (days)

## Results

- Estimated due date
- Estimated conception
- Weeks pregnant today
- Days until due date

## Formula

```
Due date = LMP + 280 days + (cycle length − 28)
Gestational age = today − LMP
```

## Explanation

Pregnancy is counted from the first day of the last period, about two weeks before conception. Only around 4–5% of babies arrive on their exact due date.

### How to use the Pregnancy Due Date Calculator

- **First day of last period**
- **Average cycle length**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Add 280 days to the last period, plus or minus the difference between your cycle and 28 days.

### Inputs explained

FieldWhat to enterFirst day of last periodA dateAverage cycle lengthIn days

### What each result means

- **Estimated due date** = [First day of last period] + 280 + ([Average cycle length] - 28)
- **Estimated conception** = [First day of last period] + 14 + ([Average cycle length] - 28)
- **Weeks pregnant today** = (today() - [First day of last period] - ([Average cycle length] - 28)) ÷ 7
- **Days until due date** = [Estimated due date] - today()

*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — First day of last period: 2026-03-01; Average cycle length: 28 days.

Results — **Estimated due date = Sun, December 6, 2026**; Estimated conception = Sun, March 15, 2026; Weeks pregnant today = 34 weeks; Days until due date = 42 days.

## Key terms

- **Naegele’s rule**: Due date = LMP + 280 days.

## FAQ

**How accurate is the due date?**
Most babies are born within two weeks either side; an early ultrasound gives a more accurate date.

**What if my cycle is irregular?**
An ultrasound dating scan is more reliable than the LMP method.

**What formula does the Pregnancy Due Date Calculator use?**
Due date = LMP + 280 days + (cycle length − 28).


## Sources

- [ACOG Committee Opinion 700 – Methods for estimating the due date](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)