Ovulation Calculator
Estimate an ovulation date and fertile window from the first day of the last period and average cycle length. Calendar dates cannot confirm ovulation.
About This Tool
Estimates ovulation as 14 days before the next expected period and displays the five preceding days through one day after as the fertile window.
People with reasonably regular cycles who want a calendar estimate. Irregular cycles, hormonal contraception, illness, postpartum changes and perimenopause can make the estimate unreliable.
All calculations happen in your browser. No data is transmitted or stored anywhere.
How the fertile window is estimated
The calculator estimates ovulation 14 days before the next expected period. It shows the fertile window from five days before through one day after that estimated date. Actual ovulation can shift even in people whose cycles are usually regular.
Authoritative references
Reviewed 23 June 2026: ACOG: fertility-awareness methods; World Health Organization: family planning and contraception.