How to Calculate Your Cycle Length
Cycle length is the number of days from the first day of one period to the first day of the next.
The simple formula
Cycle length = (start date of next period) − (start date of current period).
Example: period starts January 5, next period starts February 2 → cycle length = 28 days.
Average over several cycles
Track three to six cycles, sum the lengths, and divide by the count. This gives a realistic average and reveals variability.
What's considered normal
21–35 days. Cycles consistently shorter than 21 or longer than 35 days, or that vary by more than 7–9 days month to month, deserve a clinical conversation.