Estimate a calendar ovulation date and a broad surrounding window from the first day of a period and usual cycle length.
Estimated ovulation = period start + cycle length − 14 days. Displayed window = five days before through one day after that estimate.
For a period starting September 1 with a 28-day cycle, the estimate is September 15, with a displayed calendar window of September 10–16.
Ovulation timing varies and a regular cycle does not guarantee the estimate is right. This tool does not confirm ovulation, diagnose infertility or provide contraception. The displayed window is not a guarantee of fertile or infertile days.
References support the identified formula, unit or jurisdiction. They do not imply endorsement of this site. Check the original document for current conditions and exceptions.
Editorial responsibility: Nicolas Belotti · Methods and testing · Report an error
English edition: 16 September 2026.