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Cycle Length Calculator
Track and analyze patterns to identify irregularities early.
Medically ReviewedEvidence-BasedUpdated Feb 28, 2026
How this calculator works
This calculator uses peer-reviewed algorithms derived from clinical research on menstrual cycle physiology. It applies standard luteal phase calculations and statistical cycle analysis models to produce estimates based on your individual input data.
All calculations are performed locally in your browser. No personal data is transmitted to our servers.
Clinical Disclaimer
This tool is for educational purposes only and does not constitute medical advice. The calculations are based on statistical averages and clinical guidelines, but may not be accurate for individuals with irregular cycles or underlying health conditions. Always consult with a healthcare provider for medical advice.
Dr. Sarah Jenkins
Board Certified OB/GYN
Dedicated to advancing women's health through evidence-based medicine and patient education.
Reviewed by Dr. Emily Chen
Chief Medical Officer
Why Tracking Your Cycle Length Matters
Your menstrual cycle is often considered a "fifth vital sign." Tracking its length and variations can provide crucial insights into your overall health, hormonal balance, and fertility. It helps you anticipate your period, identify your fertile window, and spot potential health issues early.
How to Calculate Your Average Cycle Length
To calculate your average cycle length, count the days from the first day of one period to the day before your next period starts. Do this for at least three to six consecutive cycles, add the total number of days, and divide by the number of cycles tracked.
Irregular Cycles: Causes and Implications
A cycle is generally considered irregular if it frequently falls outside the 21 to 35-day range or if the length varies by more than 7 to 9 days from cycle to cycle. Causes can include:
- Polycystic Ovary Syndrome (PCOS)
- Thyroid dysfunction
- Perimenopause
- Extreme stress or weight changes
- Uterine fibroids or polyps
Improving Menstrual Health
Maintaining a balanced diet, managing stress through relaxation techniques, getting regular exercise, and ensuring adequate sleep can all contribute to a healthier, more regular menstrual cycle. If irregularities persist, consult a healthcare provider.
Sources & Citations
Last updated: Feb 28, 2026