Calculate your menstrual cycle length from two period start dates.
Normal cycle length ranges 21-35 days. The average is 28 days, but studies show fewer than 15% of women actually have a 28-day cycle. Cycles of 25-30 days are most common. Knowing your cycle length is critical for accurately predicting ovulation, since ovulation occurs approximately 14 days BEFORE your next period regardless of cycle length.
Medical Disclaimer: This tool is for educational purposes only and should not replace professional medical advice. Always consult a qualified healthcare provider.
Your average cycle length is the foundation of all period and ovulation predictions. By logging multiple cycle start dates, this calculator gives you your true average — making all future predictions significantly more accurate than using the 28-day default.
Most period apps default to 28 days — but the average woman's cycle is anywhere from 24 to 35 days. Using your personal average means your ovulation prediction could be off by days or even a week if you use the 28-day default. That's the difference between trying to conceive at the right time or completely missing your fertile window.