Cycle calculators
Five free calculators that answer the most common cycle questions. No signup, no account, runs in your browser. All five use the same underlying math; pick the one that matches what you want to know.
Calculators
- Cycle phase calculatorFree cycle phase calculator. Enter your last period and cycle length, see which of the four menstrual phases you are in today, day in phase, and what is coming next. No signup.
- Ovulation calculatorFree ovulation calculator. Estimates your ovulation day and fertile window from your cycle length and last period start. Uses the standard luteal-phase model with caveats explained.
- Luteal phase calculatorFree luteal phase calculator. Shows luteal start date, end date, and days remaining. Plus what to expect in the luteal phase and how it differs from PMS.
- Follicular phase calculatorFree follicular phase calculator. Shows follicular start date, end date, and days until it begins. Plus what your body does during follicular phase and why it matters.
- Period predictorFree period predictor. Calculates your next 3 period start dates from your cycle length and last period. Honest about model limits for irregular cycles.
Phase deep-dive references
Each phase has a reference page covering hormones, symptoms, what is normal, when to see a doctor, and embedded calculator. Use these for context; use the calculators above for quick answers.
- Menstrual phaseAn evidence-based reference on the menstrual phase: what hormones do, what symptoms are typical, what is not, and when to see a doctor. Plus an embedded cycle calculator.
- Follicular phaseAn evidence-based reference on the follicular phase: hormone pattern, cognition shifts, common variations (PCOS/PMOS, perimenopause, post-pill), and an embedded calculator.
- Ovulatory phaseAn evidence-based reference on the ovulatory phase: the 3 to 5 day window of egg release, how to detect ovulation, fertile window math, and an embedded ovulation calculator.
- Luteal phaseAn evidence-based reference on the luteal phase: progesterone biology, PMS vs PMDD, body temperature rise, common symptoms, and when to see a doctor. Plus an embedded calculator.
All calculators use the standard four-phase model with a 14-day luteal phase. Enter your last period start date and cycle length; the math runs in your browser. We do not store your cycle data on a server. Saved input persists locally on your device only and can be cleared anytime from the home page.
Calendar-based prediction is reliable for regular cycles within 1 to 3 days. It is less accurate for irregular cycles, perimenopause, PMOS (formerly PCOS), or on hormonal birth control. The individual calculator pages cover the relevant caveats for each use case.