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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.
Follow the steps given below. 1  Find the last period start date. 2  Find out shortest and longest period cycles by observing your period cycles for atleast 6 months. 3  Illustrative example of calculating fertile period.  shortest cycle=27 1  days. longest cycle=28 days.  Subtract 18 days from the shortest cycle 27-18=9 days. Subtract 10 days from the longest cycle 28-10=18 days. The 9th day to 18 th day from your last period is the unsafe period. The rest is the safe period.