Period Calculator - DoctorDK Period Calculator First Day of Last Period You can also type the date (e.g., YYYY-MM-DD). Average Cycle Length (days) Average Period Duration (days, optional) Calculate Cycle Clear Your Estimated Cycle: Estimated Next Period Start: -- Estimated Ovulation Day: -- Estimated Fertile Window: -- Current Cycle Day: -- Future Estimated Period Dates: AI Cycle Tips from DoctorDK: Fetching personalized tips...