🌱 Conception Date Calculator
Estimate when conception likely occurred — either working forward from your last period, or backward from a known or estimated due date.
Estimate from either your last period or your due date.
How the Conception Date Calculator works
From your last period, conception is estimated at roughly 14 days later (typical ovulation timing). From a due date, it's estimated as 266 days earlier, based on the standard 40-week pregnancy length.
Step-by-step calculation
- 1Enter last period date or due date.
- 2Add 14 days to the period, or subtract 266 days from the due date.
Worked example
A last period on September 1, 2026 (due June 8, 2027) points to conception around September 15, 2026.
Conception Date Calculator examples
| First day of last period | Estimated conception window |
|---|---|
| Jan 1, 2026 | January 15, 2026 |
| Mar 15, 2026 | March 29, 2026 |
| Jun 1, 2026 | June 15, 2026 |
| Sep 1, 2026 | September 15, 2026 |
| Dec 1, 2026 | December 15, 2026 |
Other inputs kept at the calculator's default values. Results calculated by this calculator.
Why use a Conception Date Calculator?
Estimate when conception likely happened.
Important factors that can affect your results
- Ovulation timing varies.
- Sperm can live up to about 5 days.
- Ultrasound dating.
Frequently asked questions
How accurate is this estimate?
It's a standard-cycle estimate — actual ovulation and conception timing varies by individual cycle length.
Which method should I use?
Use whichever date you know — your last period, or your due date — the calculator adapts accordingly.
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Last reviewed November 2026 · How we build and check our tools · Report a problem