Conception Calculator
Work backwards from a due date, or forwards from the first day of your last period, to estimate the conception date and the fertile window around it — a quick planning reference for couples tracking pregnancy timing.
How to use
- Choose whether to start from your due date or your last period.
- Enter the date (and cycle length if using LMP).
- The estimated conception date and fertile window appear instantly.
Note
From a due date, conception is estimated as due date minus 266 days (38 weeks). From LMP, ovulation is estimated as LMP plus cycle length minus 14 days. The fertile window is roughly the 5 days before ovulation through the day after. Real cycles vary, so treat these as estimates. Estimate only — not medical advice.
Frequently Asked Questions (FAQ)
What does the Conception Calculator do?
It estimates when conception happened and the fertile window around it, working backwards from a due date or forwards from your last period.
How is conception estimated from a due date?
Conception is estimated as the due date minus 266 days (38 weeks).
How is ovulation estimated from the last period?
Ovulation is estimated as LMP plus cycle length minus 14 days, and the fertile window is roughly the 5 days before ovulation through the day after.
Is this tool free?
Yes, it is free to use, runs in your browser, and needs no signup.