Conception Date Calculator
Estimate the likely conception date and a conception window from either a due date or the first day of your last period.
Independently verified for accuracy
Calculator by Toolsloft ↗- Estimated conception date
- 2026-01-15
- Conception window start
- 2026-01-12
- Conception window end
- 2026-01-19
- Last period date
- 2026-01-01
- Due date
- 2026-10-08
This calculator estimates when conception likely happened from either a due date or the first day of the last menstrual period. It returns a single most likely conception date plus a short window around it, since the exact day is rarely known. Use it to get a working estimate of timing during early pregnancy.
How this is calculated
The estimate is based on Naegele's rule: a pregnancy averages 280 days from the first day of the last menstrual period, and conception happens about 14 days after that date. Given a last period date the tool adds 14 days; given a due date it subtracts 280 days to find the last period and then adds 14. These are estimates, and a clinician's dating scan gives a more precise date.
How to use
- Choose whether you are starting from your last period date or your due date.
- Enter that date in YYYY-MM-DD format.
- Read the estimated conception date and the conception window around it.
- Check the matching last period and due date the tool derived.
Examples
- From last period 2026-01-01:
conception about 2026-01-15 - From due date 2026-10-08:
conception about 2026-01-15
FAQ
- How is the conception date estimated?
- It uses Naegele's rule. A pregnancy averages 280 days from the first day of the last menstrual period, and conception happens about 14 days after that date, so the tool works backward from a due date or forward from the last period.
- How accurate is it?
- It is an estimate built on average cycle timing, so the true conception date can fall a few days on either side. A clinician using an early ultrasound dating scan can give a more precise date.