How long is a normal menstrual cycle?

The short answer

• Cycle length: 24 to 38 days, counted from the first day of one period to the first day of the next.
• Bleeding: up to 8 days.
• Regularity: your shortest and longest cycles differ by about 7 to 9 days or less, depending on age.

These are the normal limits set by FIGO, the International Federation of Gynecology and Obstetrics, in its 2018 guidelines.

Why 28 days is the exception

A study of more than 600,000 cycles tracked in an app found an average cycle length of 29.3 days, and only about 13% of cycles were exactly 28 days long. The same study found cycles tend to get slightly shorter with age, by about 3 days on average between 25 and 45.

Most of the variation comes before ovulation. The phase after ovulation, the luteal phase, tends to be more stable. That's why confirming ovulation is one of the best ways to predict your next period.

How to work out your own cycle length

Count from the first day of real bleeding (not spotting) to the day before your next period starts. Track at least three or four cycles before drawing conclusions, since one cycle can be thrown off by illness, stress or travel.

When to talk to a clinician

• Your cycles are often shorter than 24 days or longer than 38 days
• Your cycle length varies by more than about a week from cycle to cycle
• Your periods last longer than 8 days
• You go 90 days without a period and aren't pregnant