A worked example
A last period starting June 1, 2026 with a 28-day cycle gives an estimated ovulation date of June 15, with a fertile window from about June 10 to June 16.
Frequently asked questions
Why does the fertile window start before ovulation?
Sperm can survive in the reproductive tract for up to about 5 days, so intercourse in the days leading up to ovulation can still result in conception once the egg is released — that's why the window extends backward, not just to the ovulation day itself.
How reliable is a calendar-based estimate like this?
It's a reasonable starting point, but ovulation timing varies cycle to cycle for most people — tracking methods like basal body temperature, cervical mucus changes, or ovulation predictor kits can narrow things down further.
What if my cycles are irregular?
A calendar-based estimate is less reliable with irregular cycles, since it depends on a consistent average length. Tracking actual physical signs of ovulation becomes more valuable in that case.
This calculator provides estimates for general informational purposes only and is not medical advice.