Period Cycle Calculator

Predict upcoming periods, estimate ovulation dates, identify peak fertile windows, and map safe days on a 3-month calendar.

Cycle Settings

Days between the start of one period and the start of the next (typically 28).
Number of bleeding days (typically 3-7).

Privacy Guarantee: We do not log, save, or transmit your health information. All data is processed locally on your device and disappears when you exit.

Enter the date of your last period and click Calculate to map your cycle forecast.

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Understanding Menstrual Cycles, Ovulation & Safe Windows

Explore the four key biological phases of menstrual cycles, follicular structures, and fertility mapping math.

Tracking your reproductive health starts with understanding your menstrual cycle. Our period cycle calculator provides estimates for upcoming cycle starts, fertile windows, safe days, and peak ovulation days. Standard medical parameters define a menstrual cycle as starting from the first day of bleeding to the day before the next bleeding starts. While 28 days is the statistical average, healthy cycle lengths vary between 21 and 35 days, and durations range between 3 and 7 days.

The Four Biological Phases of the Menstrual Cycle

Each cycle consists of four distinct phases governed by complex hormonal fluctuations:

  1. Menstrual Phase: The shedding of the uterine lining (endometrium), visible as bleeding. This phase represents the start of your cycle.
  2. Follicular Phase: Overlapping with menstruation, estrogen rises to trigger follicle development in the ovaries. This phase prepares an egg for release.
  3. Ovulatory Phase (Ovulation): A surge in Luteinizing Hormone (LH) triggers the release of the mature egg. This occurs approximately 14 days before the next period.
  4. Luteal Phase: Progesterone rises to prepare the uterus for potential pregnancy. If fertilization does not occur, hormone levels drop, triggering the next period.

Determining Fertile and Safe Windows

The fertile window represents the days when pregnancy is biologically possible. Sperm can live inside the female reproductive tract for up to 5 days, and a released egg lives for 12 to 24 hours. Consequently, the fertile window spans 5 days before ovulation and ends 1 day after. Days outside this window are considered "safe days" (under the standard calendar rhythm method). However, cycle variations can occur, so this tool should be used for tracking rather than contraception.

Menstrual Health FAQs

We subtract exactly 14 days (the standard luteal phase length) from the projected next period start date. For a 30-day cycle, ovulation occurs on day 16.

The calculator uses the average cycle length you input. If your cycle varies by more than 3-4 days each month, the predictions will be less precise, as menstrual cycles can respond to stress, diet, and lifestyle shifts.

We generate calendar grids for the current month and the following two months. We then highlight calculated days (period, fertile, ovulation, and safe days) based on your average cycle duration.

No. We operate under a strict zero-data policy. Your dates and parameters are stored only in the browser's temporary memory (Javascript state) and are never uploaded to any external server.