What's a Normal Menstrual Cycle and Period Length?

A normal menstrual cycle, counted from the first day of one period to the first day of the next, lasts 24 to 38 days for adults, and a normal period lasts up to 8 days. Those are the International Federation of Gynecology and Obstetrics (FIGO) definitions. The familiar 28-day cycle is an average, not a rule: most people’s cycles are a different length, and vary by a few days from one month to the next.

This article is general information, not medical advice.

FIGO’s Definitions

FIGO’s 2018 update of its systems for normal and abnormal uterine bleeding (Munro and colleagues) describes normal menstruation in non-pregnant adults of reproductive age:

Feature	Normal
Frequency (cycle length)	24 to 38 days
Regularity (shortest to longest cycle over a year)	A difference of 7 to 9 days or less, depending on age
Duration of bleeding (period length)	8 days or less
Volume	Normal for you, without interfering with your life

Cycles more often than every 24 days are frequent, and less often than every 38 days infrequent. No bleeding for 90 days or more is absent menstruation (amenorrhoea).

What Real-World Data Shows

In 2019, Bull and colleagues analysed 612,613 cycles with confirmed ovulation from 124,648 users of a fertility app:

• The mean cycle was 29.3 days.
• The follicular phase (from the period to ovulation) averaged 16.9 days but varied widely, while the luteal phase (from ovulation to the next period) averaged 12.4 days and varied less. So most of the difference in cycle length comes from when ovulation happens.
• Cycles shortened by about 0.18 days a year of age from 25 to 45.

In another study of 949 women, Johnson and colleagues found that 34% believed they had a 28-day cycle, but only 15% did.

Heavy Periods

The UK’s National Institute for Health and Care Excellence (NICE) defines heavy menstrual bleeding by its effect: blood loss that interferes with your physical, social, emotional, or material quality of life.

When to See a Doctor

• Cycles regularly shorter than 24 or longer than 38 days, or that vary a lot more than they used to.
• No period for three months when you’re not pregnant.
• Periods longer than 8 days, or heavy enough to affect your life.
• Bleeding between periods or after sex, or any bleeding after menopause.

Following It in Kaizora

On Menstrual Cycle, you record each day as you’d note it, such as flow, a period starting, and spotting. Kaizora then computes Menstrual Cycle Length (first day to the next first day) and Menstrual Period Length (the days of bleeding), as FIGO defines them; a cycle during a pregnancy has no length. The page says where you are, the day of your cycle and the range of your recent cycles, and charts cycles aligned by cycle day. It makes no predictions; see Why Period and Fertile-Window Predictions Are Often Wrong.

The Short Version

• A normal adult cycle is 24 to 38 days; a normal period lasts up to 8 days.
• Only about 15% of women in one study had a 28-day cycle.
• Most variation in cycle length comes from when ovulation happens.
• See a doctor for cycles outside 24 to 38 days, missed periods, or heavy bleeding.

Sources

• Munro MG et al., “The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions,” International Journal of Gynecology & Obstetrics 143, no. 3 (2018): 393–408. doi:10.1002/ijgo.12666
• Bull JR et al., “Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles,” npj Digital Medicine 2 (2019): 83. doi:10.1038/s41746-019-0152-7
• Johnson S, Marriott L, Zinaman M, “Can apps and calendar methods predict ovulation with accuracy?” Current Medical Research and Opinion 34, no. 9 (2018): 1587–1594. doi:10.1080/03007995.2018.1475348
• National Institute for Health and Care Excellence, Heavy menstrual bleeding: assessment and management (NG88).