# Conception Calculator

> Estimate the likely conception date and the window of intercourse that led to it, from your due date or from your last period and cycle length.

Category: Health & fitness · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/conception-calculator

A pregnancy lasts about 266 days from conception, or 280 days from the first day of the last period. Counting back from a due date, or forward from a period, estimates when conception happened.

Also called: Conception date calculator.

## Inputs
- I know my (options: Due date, Last period; default due)
- Due date (default 2026-10-12)
- First day of your last period (default 2026-01-05)
- Usual cycle length (unit days; default 28)

## Results
- Estimated conception
- Likely window of intercourse
- Due date
- Equivalent last-period date

## Formula

```
From a due date: conception ≈ due date − 266 days.  From a last period: conception ≈ LMP + (cycle length − 14);  due date = conception + 266 days.  Window = conception − 5 days to + 1 day.
```

Where:
- LMP: first day of the last menstrual period
- 266 days: average time from conception to birth
- 280 days: average time from LMP to birth (Naegele’s rule, 28-day cycle)

## Worked example
1. Conception: 12 Oct 2026 − 266 days = 19 Jan 2026.
2. Window: 5 days before (14 Jan) to 1 day after (20 Jan), because sperm can survive several days.
3. Equivalent LMP: 12 Oct 2026 − 280 days = 5 Jan 2026.

**Result:** A due date of 12 October 2026 points to conception around 19 January 2026, from intercourse most likely between 14 and 20 January.

## What the result means

The estimate is the most likely date, not a certainty. Since sperm can survive up to about a week, intercourse several days before ovulation can lead to pregnancy, which is why a window is shown.

## Assumptions and limits
- Ovulation (and conception) occurs 14 days before the next expected period.
- Pregnancy lasts an average of 266 days from conception (280 days from the last period).
- Even with an ultrasound-based due date the conception estimate is uncertain by several days; with an LMP-based date it can be off by a week or more.
- Cannot establish paternity: conception dates are too imprecise for that.
- IVF pregnancies have known transfer dates; use those instead.

## Questions

### Can this tell me the exact day I conceived?

No. Ovulation timing varies and sperm survive several days, so dates are estimates with a window of about a week.

### Why is my due date 280 days after my period, not 266?

Pregnancy is conventionally dated from the first day of the last period, about two weeks before conception in a 28-day cycle, so the two counts differ by 14 days.

## Sources
- [StatPearls (NCBI Bookshelf): Estimated Date of Delivery](https://www.ncbi.nlm.nih.gov/books/NBK536986/)
- [Tommy’s: Ovulation and fertility](https://www.tommys.org/pregnancy-information/planning-pregnancy/how-get-pregnant/ovulation-and-fertility)

## Related calculators
- [Ovulation Calculator](https://calculators.geekmidst.com/calculators/ovulation-calculator)
- [Period Calculator](https://calculators.geekmidst.com/calculators/period-calculator)
- [Date Difference Calculator](https://calculators.geekmidst.com/calculators/date-difference-calculator)

_This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/conception-calculator