Health & Fitness

Period Calculator | Next Period Date, Fertile Window & Cycle Tracker

Predict your next period date, fertile window, and ovulation day from your last period and average cycle length. Displays a 6-month forward calendar with all cycle events color-coded. Supports irregular cycles with adjustable length range. Not for contraceptive use.

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First Day of Last Period

Average Cycle Length (days)

Period Duration (days)

How Menstrual Cycle Prediction Works

This calculator uses the start date of your last menstrual period and your average cycle length to estimate future period dates, ovulation, and fertile windows. The method assumes a regular cycle — in reality, cycles vary by 2–7 days even in healthy individuals.

Understanding Your Fertile Window

Ovulation typically occurs approximately 14 days before the start of the next period (not 14 days after the period begins). The fertile window spans the 5 days before ovulation plus the day of ovulation itself, because sperm can survive in the reproductive tract for up to 5 days. The egg is viable for only 12–24 hours after release.

What Affects Cycle Length?

Stress	Can delay or advance ovulation by several days to weeks
Exercise intensity	Extreme exercise may suppress ovulation (hypothalamic amenorrhea)
Body weight changes	Significant weight changes affect hormone production
Illness	Temporary illness can shift cycle timing
Hormonal contraceptives	Some methods regulate cycles; others eliminate periods
Perimenopause	Cycles become less predictable in the 40s and early 50s

Medical Disclaimer: This tool is for informational purposes only and is not a substitute for contraception, fertility treatment, or medical advice. Menstrual cycle prediction is not reliable enough for pregnancy prevention. Consult a gynecologist or OB/GYN for concerns about your cycle or fertility.

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