A traditional method of natural family planning (also known as rhythm method). The fertile phase (the days around the ovulation when an mature egg cell is present) of the menstrual cycle must be determined by calculating the length of at least six previous menstrual cycles. To prevent pregnancy, this method requires not having sexual intercourse during the calculated fertile days of a woman's menstrual cycle. When used alone, the calendar method is very unreliable, especially for girls and women with irregular menstrual cycles, and may be overly restrictive for some couples. This method does not prevent STIs, including HIV.

a natural family planning method utilizing a calendar to track fertile days in a woman's cycle

See Rhythm method.