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1 Record the number of days in each cycle, including the day you start your period. 2  The last day of each cycle is the day before your next period starts. 3  Keep tracking your cycle this way for eight to twelve cycles. 4  The more cycles you have to reference, the more accurate the calendar method will be.
How to Calculate Menstrual Cycle Length. Menstrual cycle length is the number of days your menstrual cycle lasts. It is not the same as period length, which is the number of days your period lasts. The day your period starts is considered Cycle Day (CD) 1 -- the start of a new menstrual cycle. Your average cycle length displays. 2  Each time you track your next period, your cycle length is updated! 3  Use My History/Quick History, or the Menstrual Cycle Chart, to see your average, shortest and longest menstrual cycle length.