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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.
By doing this, you can determine how many days your cycle is from day one to day one of the next period. Do this for each cycle, then average the numbers. For example, my last four cycles were: Cycle One - 29 days, Cycle Two - 31 days, Cycle Three - 31 days, Cycle Four - 30 days. Add up the number of days, divide by 4 months, and you get an average of 30 days. With that number, you can predict the first day of your next period when adding 30 days to the last. So, if my last period started on February 2, I add 30 days and come up with March 4 for the first day of my next period.