Enter your last period date and cycle length to estimate your next period, ovulation day, and fertile window.
Note: These predictions are estimates based on the cycle length you enter. Stress, travel, illness, weight changes, and medications can all shift ovulation and period timing. For accurate tracking, use this tool consistently and consult with a healthcare provider for personalized guidance.
This calculator uses three pieces of information — your last period start date, your average cycle length, and your typical period length — to estimate your next period, ovulation day, and fertile window. It applies standard cycle math: ovulation typically occurs 14 days before your next period, and the fertile window spans 5 days before ovulation through 1 day after. While no calculator can predict your body with perfect precision, this tool gives you a reliable baseline for planning, tracking, and understanding your cycle.
Counting your menstrual cycle is simple once you know the rule: start on day 1 of your period (the first day of bleeding) and count every day until the day before your next period starts. That total number is your cycle length.
Period starts: March 1
Next period starts: March 29
Cycle length: 28 days
Do not count the first day of the next period as part of the previous cycle — it is day 1 of the new cycle. If you track 3–6 cycles and average the lengths, you get a much more accurate picture of your typical cycle than using a single month alone.
For most adult women, a normal menstrual cycle lasts between 21 and 35 days. The textbook average is 28 days, but anything within that range is considered medically normal if it is consistent for you. Teenagers often have longer or more irregular cycles in the first few years after menarche. Perimenopausal women may see cycles shorten, lengthen, or become unpredictable as hormone levels shift.
An irregular cycle means your cycle length varies by more than 7–9 days from month to month, or repeatedly falls outside the 21–35 day range. Here is how to track irregular cycles effectively:
Common causes of irregular cycles include stress, travel, illness, significant weight changes, intense exercise, PCOS, thyroid disorders, perimenopause, postpartum recovery, and starting or stopping hormonal birth control. If your cycles are consistently irregular, this calculator still works as a rough guide — just expect a wider margin of error.
These two dates are often confused, but they mark very different events in your cycle:
The gap between your period and ovulation depends entirely on your total cycle length. In a 28-day cycle, ovulation is around day 14. In a 32-day cycle, it is around day 18. In a 24-day cycle, it is around day 10. This calculator automatically adjusts the ovulation estimate based on the cycle length you enter.
Consult a healthcare provider if your cycles are consistently shorter than 21 days or longer than 35 days, if they vary by more than 7–9 days month to month, if you miss three or more consecutive periods (and are not pregnant), if you experience very heavy bleeding (soaking through a pad or tampon in under an hour), or if you have severe pain, clots larger than a quarter, or bleeding between periods. These can signal underlying conditions like PCOS, thyroid disorders, fibroids, or hormonal imbalances that are treatable once identified.
A menstrual cycle calculator estimates your next period, ovulation date, and fertile window using your last period start date and average cycle length. For most women, a normal cycle is 21–35 days.
Enter the first day of your last period, your typical cycle length, and how many days you usually bleed. The calculator counts forward from your last period to estimate your next one.
Ovulation is estimated as 14 days before the predicted next period — not 14 days after the last one. This means longer cycles push ovulation later, and shorter cycles pull it earlier.
Find your average cycle length from past period dates.
Estimate when your luteal phase starts and how long it lasts.
Find your fertile window and most likely ovulation day.
Estimate your due date adjusted for cycle length.
Track and average irregular cycles to spot your pattern.
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a healthcare provider with questions about your menstrual cycle, fertility, or reproductive health.
All the tools and guides in this cluster, in one place.
Predict your next period, fertile window, and phases.
Current pageFind your average cycle length.
OpenEstimate when your luteal phase starts.
OpenTrack irregular cycles and spot patterns.
OpenStep-by-step guide to counting cycle days.
OpenWhat's normal for cycle length.
OpenTiming and signs of the luteal phase.
Open