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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.
3. Keep a record. Note the first and last days of your period on a calendar. This way, you can an estimate when your next period might come. Most women's periods come every 28 days, but if you keep track of your periods, you can determine what length your own cycle is. 4. Use an app.