How long is a normal menstrual cycle?

The NHS describes menstrual cycles as commonly 21–35 days, with an average of around 28 days. Other guidance uses different boundaries, and a change from your usual pattern, or bleeding or pain that causes problems, still needs attention even when the length falls within a published range.

Count from the first day of your period

A period starting on 6 April, followed by the next on 11 May, makes a 35-day cycle. In this fictional example, 6 April is day 1 and 10 May is day 35. On 11 May, counting starts again at day 1.

That is how you count a menstrual cycle: from the day your period begins to the day before the next period. Count the whole interval, including days without bleeding. If you start counting only once the bleeding has stopped, you leave part of the cycle out.

Compare the ranges

Published ranges differ, and some apply specifically to teenagers. Keep the age group in mind when comparing your cycle with these figures.

Guidance	Cycle length described	How to read it
NHS: Periods	Around 28 days for most people; commonly 21–35 days	General period guidance. The average isn’t a required length.
ACOG: Abnormal Uterine Bleeding	Typically 21–35 days	The American College of Obstetricians and Gynecologists gives the same range here.
Office on Women’s Health: Your menstrual cycle	24–38 days	This US government guide uses different adult boundaries.
ACOG: Your First Period	About 28 days on average; 21–45 days can also be normal	Guidance for teenagers. Use the adult ranges when comparing adult cycles.

A 35-day cycle sits within all these ranges. It can be typical without matching the 28-day average.

The differences matter at the edges. A cycle of 36–38 days falls within the Office on Women’s Health range but beyond the NHS and ACOG adult ranges above. These pages don’t agree on one shared cut-off, and the teenage range shouldn’t be substituted for adult guidance.

The table helps you compare the numbers. As the NHS and ACOG explain, changes from your usual pattern, bleeding and accompanying symptoms also matter when deciding whether to seek help.

Bleeding days are only part of the cycle

Bleeding can last between 2 and 7 days, usually about 5, and is often heaviest during the first 2 days. ACOG’s general guidance says a menstrual period usually lasts up to 7 days.

These are the days of bleeding within a cycle. In the April-to-May example, the cycle remains 35 days long regardless of which days within it involved bleeding.

How heavily you bleed matters too. ACOG includes heavy bleeding and bleeding between periods among changes that need assessment, even without a prolonged period. The Office on Women’s Health advises seeking help when pain or bleeding disrupts ordinary activities.

If flow is your main concern, read about heavy periods and when to get help. Bleeding that causes problems deserves attention even when your cycle length is familiar.

What changes with age

In the first years after periods begin, ovulation, the release of an egg, may not happen regularly. A lack of ovulation can produce irregular and sometimes heavy bleeding, which helps explain why ACOG’s first-period guidance allows a wider interval than its general adult guide.

Long gaps in adolescence still need attention. That guidance advises seeing a doctor if periods stop for more than three months, or come more often than every 21 days or less often than every 45 days. Pain or bleeding that prevents usual activities also needs checking.

Cycles are usually more regular in your 20s and 30s. Having a regular cycle still allows some variation from month to month. It doesn’t mean every period has to begin on exactly the same schedule.

As menopause approaches, ovulation may become less consistent again. Periods can become less predictable, and bleeding can change in length or heaviness. There’s more about this stage in perimenopause and period changes. ACOG advises discussing abnormal bleeding even when changes might be expected around menopause.

When your usual pattern shifts

Cycle length can vary from month to month. A start date shifting by a few days tells you what changed, but cannot explain why.

Stress, substantial weight changes and excessive exercise are among the possible reasons for irregular periods. Hormonal contraception can also change the pattern. For these and medical explanations, read irregular periods: causes and when to see a doctor. Cycle-length calculations and calendar estimates cannot diagnose the cause.

At an appointment, it helps to describe both length and consistency. “My cycles are usually near the longer end of the range” means something different from “my previously predictable periods have changed”. Either gives your GP something to work with without you having to work out the explanation first.

Getting help for a short, long or changing cycle

The NHS advises seeing a GP when periods are irregular or your normal pattern changes. It counts gaps shorter than 21 days or longer than 35 days as irregular.

Arrange an appointment if:

• Periods keep arriving outside that range, or a previously familiar pattern changes.
• Bleeding lasts longer than 7 days.
• You bleed between periods or after sex.
• Pain or bleeding interferes with work, study or other usual activities.
• You have missed three periods in a row.

ACOG and the Office on Women’s Health give the bleeding advice above; the NHS advises a GP appointment after three missed periods in a row. Ask sooner about a worrying change.

If pregnancy is possible, take a pregnancy test according to its instructions. Most tests can be used from the first day of a missed period; there is no need to wait for several missed periods.

Pelvic pain that needs urgent or emergency assessment

Pelvic pain with unusual vaginal bleeding, or pelvic pain when you might be pregnant, needs urgent assessment. In the UK, contact NHS 111; elsewhere, use your local urgent medical service.

Pelvic pain that is severe or worsening, or comes with faintness, difficulty breathing or heavy vaginal bleeding, needs emergency care. Call 999 in the UK, or your local emergency number, or attend an emergency department (A&E in the UK).