Period Cycle Calculator

A normal menstrual cycle length ranges from 21 to 35 days, with 28 days being the often-cited 'average.' However, many healthy women have cycles outside this range. The key is consistency—if YOUR cycles are consistently 32 days, that's normal for you. Cycle length is counted from the first day of bleeding (Day 1) to the day before the next period starts. Track at least 3 cycles to establish your personal average.

Count from Day 1 of your period (first day of bleeding) to Day 1 of your next period. That number of days is your cycle length. For example: If your period started on January 1st and your next period started on January 29th, your cycle was 28 days. Our calculator automates this by comparing multiple period start dates and calculating the average, giving you a more accurate picture than tracking just one cycle.

The fertile window is approximately 6 days: the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the female reproductive tract, while the egg is viable for 12-24 hours after release. For a 28-day cycle with ovulation on Day 14, the fertile window would be Days 9-14. Our calculator estimates your fertile window based on your average cycle length by predicting ovulation 14 days before your next expected period.

Ovulation typically occurs about 14 days BEFORE your next period starts (not 14 days after your last period). This is called the luteal phase, which remains relatively constant at 14 days for most women. So for a 28-day cycle, ovulation is around Day 14. For a 32-day cycle, ovulation would be around Day 18 (32-14=18). Our calculator uses this method to estimate your ovulation date based on your average cycle length.

1. MENSTRUAL PHASE (Day 1-5): Uterine lining sheds (period bleeding). 2) FOLLICULAR PHASE (Day 1-13): Ovaries prepare follicles containing eggs; estrogen rises. 3) OVULATION (Day 14): Egg released from ovary; peak fertility. 4) LUTEAL PHASE (Day 15-28): Empty follicle becomes corpus luteum, produces progesterone; if no pregnancy, hormone levels drop and cycle restarts. Note: These days are based on a 28-day cycle—your phases will shift proportionally for longer/shorter cycles.

Irregular cycles can be caused by: 1) STRESS—affects the hypothalamus, disrupting hormone signaling. 2) SIGNIFICANT WEIGHT CHANGES—fat cells produce estrogen. 3) EXCESSIVE EXERCISE—can suppress ovulation. 4) PCOS (Polycystic Ovary Syndrome)—common hormonal disorder. 5) THYROID DISORDERS—thyroid hormones interact with reproductive hormones. 6) PERIMENOPAUSE—ovarian function declines. 7) BREASTFEEDING—prolactin suppresses ovulation. 8) STOPPING BIRTH CONTROL—cycles may take months to regulate. If irregularity persists, consult a healthcare provider.

A short cycle is less than 21 days. Occasional short cycles are common and usually not concerning. However, consistently short cycles may indicate: 1) Perimenopause (ovarian aging). 2) Thyroid disorders (hyperthyroidism). 3) Medical conditions affecting ovulation. Short cycles can also mean you ovulate earlier, affecting fertility timing. If you consistently experience cycles under 21 days, especially with heavy bleeding, consult a healthcare provider to rule out underlying conditions.

A long cycle is more than 35 days. Long cycles indicate delayed or absent ovulation. Causes include: 1) PCOS—most common cause of anovulation. 2) HYPOTHYROIDISM—underactive thyroid slows metabolism. 3) HYPERPROLACTINEMIA—elevated prolactin suppresses ovulation. 4) SIGNIFICANT STRESS—affects hormone cascade. 5) EATING DISORDERS or extreme exercise. Occasional long cycles (1-2 per year) are normal. Consistently long cycles may affect fertility and warrant medical evaluation.

Prediction accuracy depends on cycle regularity. For regular cycles (variation < 3 days month-to-month), predictions can be accurate within 1-2 days. For irregular cycles, predictions are less reliable as the body doesn't follow a fixed schedule. Our calculator shows your 'regularity score' based on cycle variation—higher scores mean more reliable predictions. Track more cycles for better accuracy. Remember: these are ESTIMATES, not medical diagnoses, and should not be used as contraception.

The luteal phase is the time from ovulation to the start of your next period. It's typically 12-14 days and remarkably consistent for each individual. A 'luteal phase defect' (less than 10 days) can cause fertility problems because there's not enough time for a fertilized egg to implant before the uterine lining sheds. If you're trying to conceive and suspect short luteal phase, track your cycle and consult a fertility specialist. Our calculator assumes a 14-day luteal phase for predictions.

NO. This calculator should NOT be used as a method of contraception. Calendar-based 'fertility awareness methods' have high failure rates (15-25% typical use) because ovulation can be unpredictable. Sperm can survive 5 days, stress can shift ovulation timing, and cycles vary. This tool is for INFORMATIONAL purposes only—to help you understand your body. For reliable contraception, consult a healthcare provider about evidence-based methods.

Track at least 3-6 periods for a meaningful average. More data = better accuracy. With just 2 periods, you get 1 cycle length, which may not be representative. With 6+ periods, you have enough data to calculate average cycle length, detect patterns, assess regularity, and make more reliable predictions. Our calculator supports up to 12 period entries for comprehensive tracking.

The regularity score (0-100%) indicates how consistent your cycle lengths are. It's based on standard deviation—the mathematical measure of variation. 90-100% (Regular): Cycles vary by 0-3 days. Predictions are highly reliable. 70-89% (Somewhat Irregular): Cycles vary by 4-7 days. Predictions are estimates. Below 70% (Irregular): Cycles vary by 7+ days. Predictions are less reliable. Higher regularity suggests predictable hormonal patterns and easier fertility planning.

PMS (Premenstrual Syndrome) refers to physical and emotional symptoms that occur 1-2 weeks before your period (during the luteal phase). Symptoms include mood swings, bloating, breast tenderness, food cravings, fatigue, and headaches. They're caused by the rise and fall of progesterone and estrogen. Symptoms typically improve once your period starts. Up to 75% of women experience some PMS; 3-8% have severe PMDD (Premenstrual Dysphoric Disorder) requiring treatment.

No. This calculator runs entirely in your browser. Your period dates are processed locally on your device and are never sent to our servers or stored anywhere. When you close or refresh the page, all data is cleared. We believe menstrual health data is private and have designed this tool with privacy-first principles. For persistent tracking, consider using a dedicated period tracking app with a clear privacy policy.