Pregnancy MetricsPregnancy & preconception calculators

Ovulation Calculator

Enter the first day of your last period and your usual cycle length to estimate ovulation, the fertile window, and your next period.

Cycle day 1 is the first day bleeding begins.

Supported model range: 21–35 whole days. This is not a diagnostic threshold or proof of regular ovulation.

Calculator inputs are processed on this device. Default calculations do not send them to a calculation server, write them into the page URL, or save them to an account or project-controlled database. Read the Privacy Policy.

How this estimate works

The calendar model estimates ovulation on cycle day (usual cycle length − 14). Its fertile window includes that estimated day and the previous five days. This is not biological confirmation, pregnancy probability, or contraception advice; uncertain or variable cycles make the estimate less dependable.

For evidence and limits, read the Trying, Ovulation and Pregnancy Test Timing Guide or the methodology and sources.