What is an ovulation calculator? Ovulation calculator is a smart tool that helps women predict their ovulation period. By entering their individual menstrual cycles and menstrual duration, the tool helps women predict the most likely days and periods of ovulation, helping them better plan their fertility or prevent pregnancy. How to accurately use an ovulation calculator? First of all, understanding your personal menstrual cycle is a prerequisite for using an ovulation calculator. Women\’s menstrual cycles vary, generally ranging from 21 to 35 days. Before using the ovulation calculator, you need to clear