Short answer

For adults, a normal menstrual cycle is about 21 to 38 days long, counted from the first day of one period to the first day of the next. ACOG's patient guidance gives 21 to 35 days in one FAQ and 24 to 38 days in another. Periods normally last up to 7 or 8 days, and in the first years after periods start, cycles of 21 to 45 days are normal.

Key facts

  • ACOG's FAQ on abnormal bleeding: a normal cycle is typically 21 to 35 days, and a normal period lasts up to 7 days.[1]
  • ACOG's FAQ on perimenopausal bleeding: a cycle is typically 24 to 38 days, and a normal period lasts up to 8 days.[2]
  • For teens, cycles of 21 to 45 days are normal, and it can take 6 years or more for cycles to become regular.[3]
  • In 612,613 app-logged cycles, the average cycle lasted 29.3 days and the average bleed lasted 4 days.[4]
  • Only 15% of 949 women in one study actually had a 28-day cycle.[5]

How do you measure your cycle length?

  1. Mark day 1: the first day of bleeding.
  2. Keep logging until your next period starts.
  3. Count the days from day 1 of one period up to the day before the next period starts. That number is your cycle length.[3]
  4. Repeat for several cycles. The gap between your shortest and longest cycle shows how regular you are.

What is a normal cycle length at different ages?

For teens, ACOG notes that cycles that are 21 to 45 days are also normal.[3] Cycles then settle in the adult range and change again as menopause approaches.

Life stageTypical cycle lengthNotes
First years after your first period21 to 45 days[3]It can take 6 years or more for cycles to become regular
Adults21 to 35 days[1] or 24 to 38 days[2], depending on the definition usedPeriods last up to 7 to 8 days
PerimenopauseBecomes more variable[6]Early transition: consecutive cycles differ in length by 7 days or more. Late transition: gaps of 60 days or more without a period

Cycle length also shifts gradually with age. In the large app study, average cycle length got about 0.18 days shorter per year between ages 25 and 45.[4]

What counts as an irregular cycle?

Doctors look at how much your cycle length changes, not only at the average. ACOG treats periods whose cycle length varies by more than 7 to 9 days as irregular, along with cycles longer than 35 days or shorter than 21 days.[1]

For PCOS assessment, the 2023 international guideline uses these thresholds once it has been 3 or more years since your first period: cycles shorter than 21 days or longer than 35 days, or fewer than 8 cycles a year. From one year after your first period, any single cycle longer than 90 days also counts as irregular.[7]

When should you see a doctor about your cycle?

Talk with a doctor or nurse if you notice any of these:

  • Bleeding or spotting between periods, or after sex.[1]
  • Periods that last more than 7 days.[8]
  • Bleeding that soaks through one or more pads or tampons every hour for several hours in a row.[8]
  • Cycles that are often shorter than 21 days or longer than 35 days, or that vary by more than 7 to 9 days.[1]
  • No period for 3 to 6 months.[1]
  • Any bleeding after menopause.[1]

If your period is late and you could be pregnant, take a pregnancy test.