Conception calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Conception is about 266 days (38 weeks) before the due date, about 14 days after the start of the last period.
- This is an estimate; the true date of conception can be a few days either side.
| Inputs | Answer |
|---|---|
| Due date: 2027-01-24 | A due date of Sunday, January 24, 2027 suggests conception around Sunday, May 3, 2026. |
| Due date: 2026-12-06 | A due date of Sunday, December 6, 2026 suggests conception around Sunday, March 15, 2026. |
| Due date: 2027-03-15 | A due date of Monday, March 15, 2027 suggests conception around Monday, June 22, 2026. |
How to use it
Working back from the due date, conception is estimated at about 266 days (38 weeks) earlier, roughly 14 days after the start of the last period. The fertile window around it is the five days before and the day of ovulation.
This is an estimate: the true date can be a few days either side, and the date of intercourse may differ from the date of conception. This is general information and an estimate, not medical advice; talk to a healthcare professional about your own situation.
Frequently asked questions
How do I find the conception date from the due date?
Subtract 266 days (38 weeks).
Is the date exact?
No, it is an estimate within a few days.
How long before the due date is the last period?
About 280 days.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
About · How we verify this · Editorial policy · Corrections
General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.