Period Calculator

Use this calculator to estimate the future period days or the most probable ovulation days.

Menstrual cycle

The menstrual cycle comprises a series of physiological changes that prepare a woman's body for a potential pregnancy. Typically it starts between ages 12 and 15 and continues until menopause, which on average occurs around 52 years old. Cycle length is measured from the first day of one bleeding episode to the first day of the next and is regulated by fluctuating hormones. While cycle length differs among women, a regular cycle is one where the shortest and longest intervals differ by no more than eight days, with the average lasting about 28 days.

During each cycle the uterine lining thickens, and an egg capable of fertilisation is produced. The egg is released from the ovary in ovulation, a window that spans roughly five days before and up to two days after release, representing the most fertile period. If fertilisation does not occur, the uterine lining is shed in a menstrual flow, and the cycle begins anew.

A menstruation, commonly called a period, is the regular discharge of blood and uterine lining that occurs as part of the cycle. Bleeding typically lasts between two and seven days. It marks the menstrual phase, which starts when the previous cycle’s egg is not fertilised. Bleeding stops during pregnancy, usually does not resume until after breastfeeding, and ends permanently at menopause, defined as a full year without vaginal bleeding, generally between ages 49 and 52.

The Period Calculator predicts which days will involve menstrual bleeding and which are most likely to be ovulation days, presenting the information in a calendar layout. Menstrual days are those with bleeding, while probable ovulation days indicate the window when ovulation is most expected.