Period Calculator

Menstrual cycle predictions start from the first day of your last period (LMP), your average cycle length (default 28 days), and average period duration (default 5 days). Ovulation typically occurs about 14 days before the next period: ovulation day = LMP + (cycle length − 14). For a 28-day cycle, ovulation falls around day 14. The fertile window spans 5 days before ovulation through ovulation day — six days total when conception is most likely.

Period days run from LMP through LMP + period duration − 1. The next period begins LMP + cycle length. PMS (premenstrual syndrome) window is estimated as 7–10 days before the next period start. Safe days — lower pregnancy probability — are days outside the fertile window and period, though this method is unreliable for birth control.

This calculator projects 12 consecutive cycles forward (~12 months), generating a table with period dates, ovulation, fertile window, and PMS for each cycle. The calendar diagram color-codes days: red for period, green for fertile window, blue for ovulation, gray for safe days. Ovulation day takes visual priority when overlapping other categories.

Cycle length varies naturally — stress, illness, travel, and hormonal changes affect timing. Regular cycles between 21–35 days are generally considered normal. Irregular cycles reduce prediction accuracy. This tool uses your average cycle length; tracking 3–6 cycles improves personal averages.

Important: This calculator provides estimates based on average cycle patterns. Actual cycles may vary. Not a substitute for medical advice or a reliable method of birth control. Consult a healthcare provider for fertility planning, contraception, or irregular cycles lasting beyond a few months.