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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.
Taking this into account, you can calculate the time of ovulation within your cycle by subtracting the length of your luteal phase from the length of your cycle. For example, if your cycle is 28 days long and your luteal phase is 12 days long, the ovulation will occur on day 16 of your cycle (28-12=16). Ovulation Calendar uses this formula to calculate your time of ovulation.