How to use Period Predictor
The period predictor forecasts your next six menstrual periods from a single input: the first day of your last period and your average cycle and period lengths. It estimates each upcoming period's start and end dates plus the five-day premenstrual window before it, so you can plan travel, events, and symptom management weeks in advance. Because it works purely from your cycle averages, predictions are most accurate for regular cycles and become approximate further into the future.
- Select the first day of your last period using the date picker.
- Set your average cycle length (21–40 days) with the slider.
- Set your average period length (2–10 days) with the slider.
- Click Predict Next Periods.
- Review the table of upcoming start dates, end dates, and PMS windows.
Your data never leaves your device — 100% private processing.
How period prediction works
A menstrual cycle is measured from the first day of one period to the first day of the next. The predictor takes your last start date and repeatedly adds your average cycle length to project future start dates, then adds your period length to estimate each end date. The premenstrual window is placed in the five days before each start. This is the same calendar method that period-tracking apps use as their baseline before they refine it with logged history, which is why entering an accurate average cycle length matters more than any other input.
Why cycles vary and when to seek advice
Cycle length is rarely identical every month — a variation of a few days is completely normal and driven by stress, sleep, travel, illness, exercise, and hormonal shifts. Larger swings, very short or very long cycles, skipped periods, or sudden changes can have medical causes such as thyroid issues, PCOS, or perimenopause. A predictor cannot diagnose any of this; it only projects averages. If your real periods consistently land far from the predictions, or your cycle changes markedly, that pattern is worth discussing with a healthcare professional.
Worked examples
28-day cycle
Inputs: Last period 1 Jan · 28-day · 5-day
Result: Next period ≈ 29 Jan through 2 Feb
31-day cycle
Inputs: Last period 1 Jan · 31-day · 4-day
Result: Next period ≈ 1 Feb through 4 Feb
Glossary
- Menstrual cycle
- The span from the first day of one period to the first day of the next.
- Cycle length
- The average number of days in your full menstrual cycle.
- Period length
- How many days menstrual bleeding typically lasts.
- PMS window
- The premenstrual days before a period when symptoms commonly appear.
Related reading
Frequently Asked Questions
Why use Period Predictor?
- Forecasts six consecutive periods from one entry rather than a single next date
- Shows each period's start and end date based on your own period length
- Marks the five-day PMS window before every predicted period for symptom planning
- Works entirely in your browser — no account, no tracking, nothing stored
Common use cases
- Check whether a period is likely to clash with a holiday or event months ahead
- Plan ahead for the premenstrual window when scheduling demanding commitments
- Estimate the next period date after coming off a tracking app
- Share an expected date range with a partner when family planning
- Spot when an actual period arrives far from the prediction, hinting at an irregular cycle
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