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Period Calculator

Estimate your next period date, ovulation, and fertile window for the next 12 cycles.

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Tip: your entries are never placed in the page URL.
The first day bleeding started for your most recent period.
Days from the start of one period to the start of the next. Typical: 21–35.
How many days bleeding typically lasts. Typical: 2–7.
Note: Estimates assume regular cycles. Irregular cycles, stress, illness, and other factors can shift dates. Consult a healthcare provider for concerns.
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About the Menstrual Cycle

The menstrual cycle is counted from the first day of one period to the first day of the next. It typically ranges from 21 to 35 days, with 28 days being average. The period (menstrual phase) is the bleeding portion, usually lasting 2 to 7 days.

How this calculator works

The tool projects future period start dates by adding your cycle length repeatedly. It also estimates ovulation (~14 days before the next period) and the fertile window for each cycle. 12 future cycles are shown for long-range planning.

Cycle phases

Each cycle has several phases: the menstrual phase (bleeding), the follicular phase (where an egg matures), ovulation (egg release), and the luteal phase (after ovulation until the next period). Hormones like estrogen and progesterone drive these changes.

When cycles are irregular

A cycle is considered irregular if it varies by more than about 8 days from month to month, or falls outside the 21–35 day range. Stress, illness, significant weight changes, and certain medical conditions can affect regularity. If your cycles are consistently irregular, consult a healthcare provider.