Period Calculator

The textbook cycle is 28 days, but 21–35 is normal for adults, and a few days of variation month to month is expected. Projections here simply add your average cycle length — three cycles of tracking makes the average trustworthy.

Cycles consistently shorter than 21 days, longer than 35, bleeding beyond 7 days, or sudden pattern changes are worth a clinician's look — as is a missed period with a negative test repeated a week later.