An ovulation calculator estimates a woman's fertile days in a month based on the dates of menstruation. The calculator estimates that ovulation occurs around day 14 of a 28-day cycle, explains WebMD. Ovulation may occur later or earlier, depending on the length of a woman's cycle.
Women who hope to become pregnant should use ovulation calculators in conjunction with other measures of fertility, according to WebMD. Many women have cycles greater or less than 28 days, and ovulation may occur from day 11 to day 21. Women trying to track their fertility should take the their basal body temperature each morning. A rise of 0.5 to 1 degree, cervical mucus, and bloating indicate ovulation has started.