How Long Is a Normal Menstrual Cycle?

How do you measure the length of a cycle?

Cycle length is the number of days from the first day of one period up to, but not including, the first day of the next. The NHS puts it simply: day 1 is the first day of your period, and the last day of the cycle is the day before your next period begins. Note that it is the gap between period starts that counts. How many days you bleed is a separate measure.

To find your own number, write down the start date of each period for a few months and count the days between them. If you’d rather not count by hand, the cycle length calculator does it from your dates.

What range do doctors consider typical?

Different health bodies draw the line in slightly different places:

• ACOG says the menstrual cycle typically lasts between 21 and 35 days, with bleeding that generally lasts up to 7 days.
• The NHS describes the average gap between periods as around 28 days and calls periods irregular when that gap is under 21 or over 35 days.
• The US Office on Women’s Health (OWH) describes periods as regular if they usually come every 24 to 38 days.

The ranges overlap almost completely. Either way, the takeaway is the same. “Normal” is a range of about two weeks, not one fixed number.

How common is a 28-day cycle, really?

Twenty-eight days is the textbook figure because it is close to the average, not because most people have it. Large datasets from cycle-tracking apps show how wide the spread is:

• In a 2019 study of 612,613 ovulatory cycles from 124,648 app users, Bull and colleagues found an average cycle length of 29.3 days. Only 13% of cycles were exactly 28 days.
• The Apple Women’s Health Study (Li and colleagues, 2023) analyzed 165,668 cycles from 12,608 participants in the US. The average was 28.7 days and the median 28. Half of all cycles fell between 26 and 30 days, and 90% fell between 22 and 38 days.

So if your cycle is 26 or 31 days, you are in very good company.

How does cycle length change with age?

Cycle length shifts over the reproductive years in a fairly predictable way. Cycles are longer and less predictable in the teen years, become shorter and steadier through the 20s and 30s, then grow more variable again as menopause approaches.

The Apple Women’s Health Study compared each age group with people aged 35 to 39, who had the steadiest cycles. After adjusting for other factors, the differences looked like this:

Age group	Average cycle length compared with ages 35–39	Cycle-to-cycle variability compared with ages 35–39
Under 20	1.6 days longer	45% higher
20–24	1.4 days longer	37% higher
25–29	1.1 days longer	25% higher
30–34	0.6 days longer	13% higher
35–39	Reference group	Reference group (lowest)
40–44	About 0.5 days shorter	6% higher
45–49	About 0.3 days shorter	44% higher
50 and over	2.0 days longer	200% higher

Source: Li et al., npj Digital Medicine, 2023 (Apple Women’s Health Study), adjusted estimates.

The 2019 study found the same pattern from another angle: between ages 25 and 45, average cycle length fell by about 0.18 days per year. Almost all of that came from the first half of the cycle, before ovulation, getting shorter. You can read more about that in the phases of the menstrual cycle.

What about teenagers?

ACOG says that in the first years after periods begin, cycles of 21 to 45 days are typical, and that it may take six years or more for cycles to settle into a regular pattern. OWH adds that cycles longer than 38 days are common for a few years after the first period.

And in the years before menopause?

In the 40s, cycles often start to change. OWH says periods may stop for a month or a few months and then return, and may be shorter, longer, lighter or heavier than before. ACOG notes that skipping periods is common during this transition.

What counts as a regular cycle?

Regular does not mean identical. ACOG describes cycles as irregular when their length varies by more than 7 to 9 days, so a few days of difference from month to month falls within the usual range. In the Apple Women’s Health Study, the average variation within one person’s cycles was about 4 to 6 days in most age, ethnicity and BMI groups.

Cycle length also differs between groups. In the Apple Women’s Health Study, cycles were on average 1.6 days longer for Asian participants and 0.7 days longer for Hispanic participants than for white participants. Participants with a BMI of 40 or more had cycles 1.5 days longer than those with a BMI between 18.5 and 25. These are averages across thousands of people, and your own pattern is the most useful reference you have.

What can change the length of a cycle?

The NHS lists several everyday reasons a cycle may be shorter or longer than usual:

• puberty and the years leading up to menopause
• hormonal contraception, such as the progestogen-only pill, the injection or the hormonal coil (IUS)
• losing or gaining a lot of weight
• stress and anxiety
• exercising a lot
• pregnancy

If your pattern changes for one cycle and then settles back, that is common. A change that lasts several months is worth noting down.

When is it worth talking to a doctor?

Tracking is not about hitting a target number, but some patterns are worth a professional’s view. ACOG suggests talking to a doctor if you have:

• cycles shorter than 21 days or longer than 35 days
• cycle lengths that vary by more than 7 to 9 days
• no period for 3 to 6 months
• bleeding that lasts more than 7 days, or bleeding between periods or after sex

For teenagers, ACOG’s advice is to see a doctor if periods come more often than every 21 days or less often than every 45 days, or if they were regular and then became irregular for several months. OWH also suggests talking to a doctor or nurse if you go 90 days without a period and are not pregnant or breastfeeding.

Bringing a few months of start dates makes the appointment much easier. Lunation keeps that history for you: it predicts your next period from the cycles you log, works without an account, and never shares or sells your health data.