Formula
What this menstrual cycle calculator does
Predicts your next 6 periods, the fertile window, ovulation day, and PMS window from three inputs: when your last period started, your average cycle length, and how many days you typically bleed. The calculator gives you a calendar view of the next ~6 months of cycles based on simple arithmetic. It's a prediction tool for awareness and planning - it's NOT a contraception method (the rhythm method has a 10-20% failure rate) and NOT a medical diagnostic. For irregular cycles, fertility planning, or contraception, talk to a doctor.
The four phases of the menstrual cycle
(1) Menstrual phase (days 1 to 5): the period itself; the uterine lining sheds. (2) Follicular phase (days 1 to 13 in a 28-day cycle): the body prepares an egg; estrogen rises. This phase has the most variable length across people and cycles. (3) Ovulation (day 14 in a 28-day cycle): the egg is released; you're most fertile. The egg survives about 24 hours; sperm can survive up to 5 days, so the fertile window is wider than the ovulation day. (4) Luteal phase (days 15 to 28): the body prepares for pregnancy; progesterone rises. If no fertilization, hormone levels drop and the next period begins. The luteal phase is fairly consistent at 14 days; that's why ovulation = next period - 14, not last period + 14.
Tracking tips for better accuracy
Log your period start date every month for at least 3 cycles before trusting the prediction - one cycle isn't enough data. Note any symptoms that might shift the cycle: stress, illness, travel, intense exercise, weight change, new medication. For higher-precision ovulation timing (useful for conception or contraception), pair this calculator with one of: (a) basal body temperature - a sustained 0.5 to 1.0 degree F rise the morning after ovulation; (b) cervical mucus changes - egg-white consistency near ovulation; (c) ovulation predictor kits - urine LH strips that detect the LH surge 24 to 36 hours before ovulation. Many cycle-tracking apps combine these signals - this calculator is meant for quick prediction without an app.
When cycles are irregular
A cycle length is 'irregular' if it varies by more than 7 to 9 days from month to month. Common causes: PCOS (polycystic ovary syndrome), thyroid disorders, perimenopause, hormonal birth control discontinuation, extreme stress, eating disorders, breastfeeding, recent pregnancy. If you've had irregular cycles for more than a year, or if cycles suddenly become irregular after years of being regular, see a gynecologist or family doctor. Irregular cycles are common and usually treatable, but they sometimes signal underlying conditions worth checking. For prediction in irregular cycles, this calculator's output is approximate at best - use a tracking app that adapts to your real data, or use symptom-based fertility awareness instead of date-based.
Period predictor vs ovulation calculator
This calculator focuses on PERIOD prediction - when your next bleeding starts and ends. Our separate Ovulation Calculator focuses on FERTILE WINDOW - the best days to conceive (or to avoid sex if avoiding pregnancy). The math overlaps: the ovulation day appears in both. Use this one for period planning (when to stock supplies, schedule a vacation, expect PMS). Use the ovulation calculator for fertility planning (when to try to conceive). Neither replaces medical advice.
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