# Free online pregnancy due date calculator

> This calculator estimates your due date from your last period or conception date, and tells you how many weeks along you are and which trimester you're in.

Estimate your due date using Naegele's rule from the first day of your last period or conception date. Shows gestational week, trimester, days remaining, and progress.

**Interactive version:** https://onlinecalculator.me/health/pregnancy/
**Category:** Health

## How to use

1. Select your input method — LMP (last menstrual period) or conception date.
2. Enter the date.
3. If using LMP and your cycle differs from 28 days, adjust the cycle length.
4. See your estimated due date, current week, trimester, and days remaining.

## The formula: Naegele's rule

```
EDD = LMP + 280 days + (cycle length − 28 days)
```

- LMP: first day of last menstrual period
- 280 days: standard gestational length (40 weeks)
- Cycle adjustment: adds or subtracts days for cycles that differ from the assumed 28-day cycle

From a conception date, the calculator first estimates LMP as conception − 14 days, then applies the rule.

## Worked example

LMP: July 18, 2025. Cycle: 28 days.

- EDD: July 18 + 280 days = **April 24, 2026**
- If checked on October 1, 2025: gestational days = 75 → **Week 10, Day 5**
- Trimester: **First** (week 10 < week 13)
- Days remaining: 280 − 75 = **205 days**

## Notes

- Source: Naegele FC. *Lehrbuch der Geburtshilfe*. 1830. Mittendorf-Williams cycle adjustment from *Obstet Gynecol* 1990;75(6):929-933.
- ACOG recommends ultrasound dating in the first trimester for greater accuracy (Committee Opinion 700, 2017).
- This tool is for informational planning — consult your healthcare provider for medical guidance.

## Frequently asked questions

### How is the due date calculated?

The calculator uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If you enter a conception date, the calculator estimates LMP as 14 days before conception, then applies the same formula.

### What is gestational age?

Gestational age counts from the first day of the last menstrual period, not from conception. This means week 1 of pregnancy begins before conception occurs. Full term is 40 gestational weeks.

### What are the trimesters?

First trimester: weeks 1–13 (up to week 13). Second trimester: weeks 14–27. Third trimester: weeks 28–40.

### What is the cycle-length adjustment?

Naegele's rule assumes a 28-day cycle. If your cycle is longer or shorter, the calculator adjusts by adding the difference from 28. For example, a 32-day cycle adds 4 days to the due date.

### How accurate is this estimate?

Only about 5% of babies are born exactly on the estimated due date. Most full-term babies arrive within two weeks before or after. The estimate is more accurate with ultrasound dating in the first trimester.

## Sources

- [Estimated date of delivery (Naegele's rule)](https://en.wikipedia.org/wiki/Estimated_date_of_delivery) — Wikipedia
- [Gestational age](https://en.wikipedia.org/wiki/Gestational_age) — Wikipedia

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