Period & Menstrual Cycle Calculator
Predict your next period start date, track hormonal cycle phases, and forecast future menstrual schedules with 100% private in-browser tracking.
Cycle Phase Breakdown
Next 6 Cycles Schedule Forecast
| Cycle | Expected Period Dates | Estimated Ovulation | Cycle Length |
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How the Menstrual Cycle is Calculated
The menstrual cycle begins on the very first day of your period (Day 1 of bleeding) and ends on the day immediately before your next period begins. While standard medical textbooks cite 28 days as average, healthy cycles range from 21 to 35 days.
For deeper analysis and related planning, you can also explore our BMI Calculator and BMR Calculator.
Estimated Ovulation = Next Period Start - 14 Days
Premenstrual Phase (PMS) = Days 24 to 28 of Cycle
The 4 Stages of the Hormonal Cycle
- Menstrual Phase (Days 1 to 5): The uterine lining sheds when pregnancy has not occurred. Estrogen and progesterone are at baseline levels.
- Follicular Phase (Days 6 to 13): Follicle-stimulating hormone (FSH) stimulates the ovaries to prepare an egg. Estrogen rises, boosting energy and mood.
- Ovulatory Phase (Day 14): A surge in luteinizing hormone (LH) triggers the release of the mature egg. High libido and energy are typical.
- Luteal Phase (Days 15 to 28): Progesterone peaks to maintain the uterine lining. If conception does not occur, hormone levels drop, leading to premenstrual symptoms (cramps, fatigue, bloating).
Frequently Asked Questions
How many days is a normal menstrual cycle?
The standard medical definition of a normal menstrual cycle is between 21 and 35 days, with an average length of 28 days from the first day of one period to the first day of the next.
What are the four main phases of the menstrual cycle?
The four phases are: 1. Menstruation (bleeding phase, days 1-5), 2. Follicular phase (egg matures, days 1-13), 3. Ovulation (egg release, around day 14), and 4. Luteal phase (uterine lining thickens, days 15-28).
Why does my period arrive on different dates each month?
A calendar month has 30 or 31 days (except February), while average menstrual cycles are 28 days. Because cycle lengths do not match calendar months, period dates shift forward by 2 to 3 days each calendar month.
Is my menstrual tracking data stored or transmitted?
No. Calc369 processes all calculations strictly inside your web browser. No personal health information or cycle dates are sent to any remote server or third party.