How cycle prediction works

This calculator uses your last period start date and average cycle length to predict future cycle events. The average menstrual cycle is 28 days, but cycles from 21 to 45 days are considered normal.

The four cycle phases

Menstrual phase (days 1–5)

Menstruation occurs. Hormone levels (estrogen, progesterone) are at their lowest.

Follicular phase (days 1–13)

Estrogen rises as follicles in the ovaries mature. Energy and mood typically improve. This phase overlaps with menstruation.

Ovulation (around day 14)

A mature egg is released from the ovary. This is the most fertile period of the cycle. Ovulation typically occurs approximately 14 days before the next period.

Luteal phase (days 15–28)

Progesterone rises to prepare the uterus for potential implantation. If fertilization does not occur, hormone levels drop and menstruation begins.

Fertile window

The fertile window spans approximately 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive in the reproductive tract for up to 5 days. This is an estimate, and conception may be possible around the day after predicted ovulation; these dates cannot identify “safe days” or be used as contraception.

Tracking irregularities

A cycle is considered irregular if it varies by more than 7–9 days from one month to the next. Common causes include:

Stress: elevated cortisol can suppress GnRH, delaying or skipping ovulation.
Weight changes: significant weight loss or gain affects estrogen production and cycle regularity.
Thyroid disorders: both hypothyroidism and hyperthyroidism can cause irregular cycles.
Polycystic ovary syndrome (PCOS): the most common cause of irregular cycles in reproductive-age people; characterized by elevated androgens and anovulatory cycles.
Perimenopause: cycles become irregular in the years leading up to menopause, typically starting in the mid-to-late 40s.

If cycles are consistently irregular, shorter than 21 days, or longer than 45 days, consult a healthcare provider for evaluation.

Using this tool effectively

Track 3–6 period start dates elsewhere, then enter your best average cycle length here. This tool does not store cycle history or calculate an average from past dates. Cycle length can naturally vary from month to month, so predictions are estimates rather than precise dates.