How long is a normal menstrual cycle? What's regular, and what isn't

In short: a normal menstrual cycle lasts anywhere from 21 to 35 days, counted from the first day of one period to the day before the next (NHS). Twenty-eight days is the number everyone quotes, but it's an average, not a rule: in a study of more than 612,000 cycles, only 13% were exactly 28 days long, and the average was 29.3 (Bull et al., 2019). Your period itself usually lasts 2 to 7 days (NHS).

How to count your cycle

Day 1 is the first day of real bleeding, not spotting. Your cycle ends the day before your next period starts. So if your period started on 1 March and the next one on 30 March, that cycle was 29 days long.

One cycle doesn't tell you much. Count three or more in a row and you'll see your own range, which is far more useful than any textbook number. If you know your last period date and roughly how long your cycles run, our free period calculator will estimate your next three.

What's a normal cycle length?

• Cycle length: 21 to 35 days is considered normal for adults (NHS, Cleveland Clinic).
• Period length: 2 to 7 days of bleeding, usually around 5, heaviest in the first two days (NHS).
• Blood loss: about 20 to 90 ml over a whole period, roughly 1 to 5 tablespoons, though some people lose more (NHS).

If your periods only started recently

When periods first begin, usually around 12, it's normal for cycles to be longer, less predictable or heavier. It can take up to three years for them to settle into a pattern (Cleveland Clinic, NHS).

How much can it change month to month?

Some. Regular doesn't mean identical. In the same large study, one person's cycle length typically varied by a couple of days or so from month to month (Bull et al., 2019).

Most of that variation comes from the first half of the cycle, before ovulation. The follicular phase averaged 16.9 days but ranged from about 10 to 30, while the luteal phase after ovulation was steadier, averaging 12.4 days (Bull et al., 2019). So when a cycle runs long, it's usually because ovulation came later.

Cycles also tend to get a little shorter with age: by about 0.18 days a year between 25 and 45, in the same study. Later, in the years before menopause, they often become less regular before they stop (NHS).

What counts as irregular?

The NHS calls periods irregular if the gap between them is less than 21 days or more than 35 days (NHS). Cleveland Clinic also counts these as signs worth noticing (Cleveland Clinic):

• no period for 90 days or longer
• bleeding for more than 7 days
• flow that's much heavier or lighter than usual for you
• severe pain, cramping, nausea or vomiting with your period
• bleeding or spotting between periods

Common causes of irregular cycles include stress, big changes in weight, a lot of exercise, hormonal contraception, PCOS (which the NHS now calls PMOS), thyroid problems, puberty and perimenopause (NHS). We go through these one by one in why is my period late?

When to see a doctor

Irregular periods aren't always a sign of a problem, and they don't always need treatment. But see a GP or doctor if (NHS, NHS):

• your periods are irregular, or have become irregular
• your periods last longer than 7 days
• you've missed three periods in a row
• irregular periods come with weight gain or loss, tiredness, extra facial or body hair, or dry or oily skin
• you bleed between periods
• your periods are irregular and you're finding it hard to get pregnant

Bring your dates if you have them. A few months of logged cycles helps a doctor far more than "they're all over the place".

Know your own normal

The most useful number isn't 28. It's yours. Periodiva learns your cycle from what you log, so its predictions follow your body, not a textbook. Until the app launches, try the free period calculator or the ovulation calculator. Both run on your device and save nothing.

Read next: why is my period late? Or see how the month fits together: the four phases of your cycle.