Menstrual Cycle Tracker
Predict your next period, fertile window, ovulation day, and current cycle phase. See a 3-month color-coded calendar — entirely in your browser with zero data sent anywhere.
How Menstrual Cycle Tracking Works
Menstrual cycle tracking is the practice of recording the start and end of your period each month to identify patterns, predict future cycles, and understand your reproductive health. This tracker uses the calendar method — a straightforward calculation based on three data points: your last period start date, your average cycle length, and your average period duration.
The key formula: ovulation day falls approximately 14 days before your next expected period. For a 28-day cycle that started on March 1, ovulation would be around March 15. The fertile window then spans the 5 days before ovulation plus ovulation day itself — giving a 6-day window each cycle when conception is biologically possible.
Why Track Your Cycle Privately
Period tracking apps have faced criticism for sharing or monetizing deeply personal health data. This tool was built as a privacy-first alternative. Every calculation happens locally in your browser using JavaScript — no data is transmitted, no account is created, and no cookies track you across sessions. Your cycle data never leaves your device.
This matters because menstrual health information is among the most sensitive personal data you can share. Using a browser-based, offline-capable tracker means you stay in full control.
Understanding Your Cycle Phases
A complete menstrual cycle has four distinct phases, each driven by different hormones:
- Menstrual phase (days 1–5 avg): The uterine lining sheds. Estrogen and progesterone are at their lowest.
- Follicular phase (days 1–13 avg): Overlaps with menstruation. Follicle-stimulating hormone (FSH) rises, estrogen builds, an egg follicle matures.
- Ovulation phase (around day 14): A surge in luteinizing hormone (LH) triggers egg release. This is your most fertile moment.
- Luteal phase (days 15–28 avg): Progesterone rises to prepare the uterine lining. If the egg is not fertilized, progesterone drops and your period begins.
The tracker detects which phase you are currently in based on today's date and your last period start date, so you always know where you are in your cycle.
Tips for Tracking Irregular Periods
If your cycle length varies by more than 7 days from month to month, you have an irregular cycle. This is common and can be caused by stress, significant weight changes, intense exercise, thyroid conditions, PCOS (polycystic ovary syndrome), or perimenopause. A few tips for irregular cycles:
- Use the average of your last 3 cycles as your "cycle length" input for a better prediction.
- Combine calendar tracking with basal body temperature (BBT) charting — your temperature rises slightly after ovulation.
- Ovulation predictor kits (OPKs) detect the LH surge 24-36 hours before ovulation and are more reliable than calendar methods for irregular cycles.
- If your cycle is consistently shorter than 21 days or longer than 35 days, or if you skip multiple cycles, speak with a healthcare provider.