What Is the Average Menstrual Cycle Length?
The textbook average cycle is 28 days, but real cycles vary widely. Anywhere from 21 to 35 days is considered normal in adult women.
By the numbers
Large studies show the typical adult woman's cycle averages 27–29 days, with most cycles falling between 24 and 38 days.
Cycles tend to be longer and more variable in teens and in the years approaching menopause.
What affects cycle length
Stress, sleep deprivation, intense exercise, low body weight, PCOS, thyroid issues, perimenopause, and hormonal birth control all influence cycle length.
When to see a clinician
Cycles under 21 days or over 35 days, missed periods (without pregnancy), or sudden changes in pattern that persist for 2–3 cycles.