# Conception Date Calculator

URL: https://speedycalculator.com/calculators/health/conception-date/
Last updated: 2026-10-30

> The conception date calculator estimates when you conceived from your due date or the first day of your last period and cycle length, and shows the fertile window in which intercourse most likely led to the pregnancy.

## Inputs

- I know my…
- First day of your last period
- Average cycle length (days)
- Due date

## Results

- Most likely conception date
- Possible intercourse dates from
- Possible intercourse dates to
- Due date

## Formula

```
Conception ≈ LMP + (cycle length − 14)  or  due date − 266 days
Fertile window = ovulation − 5 days to ovulation + 1 day
```

## Explanation

Conception usually happens within 24 hours of ovulation. Because sperm survive up to five days, the pregnancy may have started from intercourse several days earlier, which is why the calculator shows a window.

### How to use the Conception Date Calculator

- **I know my…**
- **First day of your last period**
- **Average cycle length**
- **Due date**

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

### How it works

Choose whether you know your due date or your last period, and enter it.

### Inputs explained

FieldWhat to enterI know my…Choose one: Last menstrual period, Due dateFirst day of your last periodA dateAverage cycle lengthIn daysDue dateA date

### What each result means

- **Possible intercourse dates from** = [Most likely conception date] - 5
- **Possible intercourse dates to** = [Most likely conception date] + 1
- **Due date** = [Most likely conception date] + 266

*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 — I know my…: Last menstrual period; First day of your last period: 2026-08-26; Average cycle length: 28 days.

Results — **Most likely conception date = Wed, September 9, 2026**; Possible intercourse dates from = Fri, September 4, 2026; Possible intercourse dates to = Thu, September 10, 2026; Due date = Wed, June 2, 2027.

## Key terms

- **Ovulation**: Release of an egg from the ovary.
- **Fertile window**: Days in a cycle when intercourse can lead to pregnancy.

## FAQ

**How accurate is a conception date estimate?**
Usually within a few days; an early ultrasound helps narrow it down.

**Can a due date tell me who the father is?**
Not reliably – conception windows overlap, and only a DNA test is conclusive.

**What is the fertile window?**
The five days before ovulation plus the day of ovulation.


## 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)
- [Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med, 1995](https://pubmed.ncbi.nlm.nih.gov/7477165/)