Period/Menstrual Cycle Calculator
Understanding your menstrual cycle helps you anticipate your period, identify your fertile window, and recognize any irregularities. Enter your cycle details to see your current cycle phase, predicted next period, and estimated ovulation day.
The menstrual cycle consists of four main phases: menstruation (the period), the follicular phase, ovulation, and the luteal phase. A typical cycle lasts 28 days, but normal cycles can range from 21 to 35 days in adults. The cycle is counted from the first day of menstrual bleeding.
During the follicular phase (days 1-13 in a 28-day cycle), follicle-stimulating hormone (FSH) causes an egg to mature in the ovary. At ovulation (approximately day 14), the mature egg is released. The luteal phase follows (days 15-28), during which progesterone prepares the uterine lining for potential implantation. If fertilization does not occur, progesterone drops and the lining sheds as menstruation.
The luteal phase is relatively constant at 12-16 days (average 14), while the follicular phase varies more widely between individuals and cycles. This is why ovulation does not always occur on day 14 and why this calculator uses the cycle length to estimate ovulation by counting back 14 days from the expected next period. Tracking your cycle over several months improves prediction accuracy.