Period Calculator
Enter your last period and cycle length to predict your next six periods, so you can plan ahead. We also mark an estimated ovulation date for each cycle.
Predictions assume a regular cycle. Track a few months to learn your own pattern.
How periods are predicted
Period n starts = last period + (n x cycle length)
Estimated ovulation = period start + cycle length - 14
We project the next six periods. Most cycles run 21 to 35 days; stress, travel and hormones can shift timing, so treat these as estimates.
Source: standard menstrual cycle guidance (ACOG).
Frequently asked questions
What is a normal cycle length?
Most cycles run 21 to 35 days, counted from the first day of one period to the first day of the next. Some variation month to month is normal.
Why do my periods shift around?
Stress, travel, illness, weight change, exercise and hormones all nudge cycle timing. Calendar predictions are estimates, so track a few months for your own pattern.
When should I talk to a doctor?
See a clinician for very heavy bleeding, cycles shorter than 21 or longer than 35 days, or big unexplained changes. A period planner makes those patterns easy to show.
What is the difference between a period and the fertile window?
Your period is the bleeding phase at the start of the cycle. The fertile window is around ovulation, roughly mid-cycle. Use the ovulation calculator for that.
Medical disclaimer: This calculator is for general information and education only. It is an estimate, not medical advice, diagnosis or treatment. Always talk to your doctor or midwife about your health and any decisions.