The calendar method predicts ovulation by looking at a woman's menstrual history. Ideally, the calendar method should be used in conjunction with other predictive strategies, like observing cervical fluids, changes in the position of the cervix, and BBT charting. Also, the calendar method can be used to determine when a woman should start using ovulation tests - products that detect her "LH Surge".
For a more comprehensive calendar method, consider creating a Fertility Chart: using several predictive methods together to pinpoint your most fertile window for conceiving.
The calendar method is designed to predict general trends based on past menstrual patterns - and is therefore limited in terms of "pinpoint accuracy". The more regular a woman's cycle, the more effective the calendar method is - and perceiving ovulation patterns begins to crystallize after a few months of maintaining records.
With the ovulation calendar method, a written record is kept using a calendar to follow the patterns of each cycle. Each cycle begins with the first day of one's menstrual period and ends with - but does not include - first day of the next (which should be recorded as the first day of the next cycle).
How Chart and Read Your Ovulation Calendar
The day menstrual flow begins is 'Day One'. Circle this date on the calendar and notate as 'Day One'. For each following month, circle Day One and continue this for for at least 7-8 months.
Continue maintaining a record of the number of days in each cycle. When bleeding starts, circle the date on your calendar.
To determine the first day you are likely to ovulate, examine your records from previous months, find the shortest cycle, and subtract 18 from the total number of days.
For example, if your shortest cycle is 28 days long, subtract 18 from 28, which leaves 10. Starting with the date you circled (Day One, the first day of your current cycle) count ahead ten days and draw an O (for 'ovulation') through that second date. This day will be the date you are most likely to become fertile.
Again, the calendar method of ovulation prediction is best used in concert with other methods discussed in the Fertility Charting section - or with ovulation predictor kits.