Online Period Calculator

Online Period Calculator is a free web tool for estimating cycle dates. Enter the first day of your last period and your average cycle length, or the start dates of a few recent periods if you aren't sure, and it estimates your next period, your likely ovulation day about 14 days before it, and your fertile window, with a forecast of 3, 6 or 12 months. An irregular-cycle mode works from your shortest and longest cycles and shows an uncertain range. Results can be logged, printed, saved as a PDF or shared by link. The same site has 14 calculators, including late period, ovulation, conception, implantation and due date tools, plus guides. Dates are calculated and stored in your browser, not sent to the site's server or included in ad requests. The site shows Google AdSense ads on every page, and states that its results are estimates, not a diagnosis.