A free, browser-based period calculator that predicts your next periods, ovulation day, and fertile window based on your last cycle date and average cycle length. It uses a standard luteal-phase model, estimating ovulation as 14 days before the next expected period and projecting multiple future cycles for planning. The tool explains key cycle phases, including menstrual, follicular, ovulation, and luteal stages, and highlights fertile days when conception is most likely. All calculations run locally in the browser with no data stored or sent to a server, ensuring full privacy and instant results for cycle tracking and fertility planning.