--- title: "How long is a normal menstrual cycle?" description: "How to count your cycle, how the NHS and ACOG ranges compare, and when a short, long or changing cycle is worth raising with your GP." url: "https://myruva.co/learn/how-long-is-a-normal-menstrual-cycle" language: "en-GB" topic: "Your cycle" published: "2026-10-18" updated: "2026-10-18" author: "Ruva team" reading_minutes: 5 --- # How long is a normal menstrual cycle? How to count your cycle, how the NHS and ACOG ranges compare, and when a short, long or changing cycle is worth raising with your GP. *Written and reviewed by the Ruva team · Updated 2026-10-18 · Your cycle* The NHS says menstrual cycles are commonly 21–35 days, with an average of around 28 days. Other guidance uses different limits. Even when your cycle length falls within a published range, a change from your usual pattern still needs attention. So does bleeding or pain that causes problems. ## Count from the first day of your period Say one period starts on 6 April and the next starts on 11 May. That makes a 35-day cycle. In this made-up example, 6 April is day 1 and 10 May is day 35. On 11 May, you start again at day 1. That is how you [count a menstrual cycle](https://www.nhs.uk/symptoms/irregular-periods/): from the day your period starts to the day before the next one. Count every day in between, including days without bleeding. If you only start counting once the bleeding stops, you leave part of the cycle out. ## Compare the ranges Published ranges differ, and some are only for teenagers. Keep the age group in mind when you compare your cycle with these numbers. | Guidance | Cycle length given | What it means | | --- | --- | --- | | [NHS: Periods](https://www.nhs.uk/conditions/periods/) | Around 28 days for most people, commonly 21–35 days | General period guidance. The average isn't a length you have to match. | | [ACOG: Abnormal Uterine Bleeding](https://www.acog.org/womens-health/faqs/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](https://womenshealth.gov/menstrual-cycle/your-menstrual-cycle) | 24–38 days | This US government guide uses different limits for adults. | | [ACOG: Your First Period](https://www.acog.org/womens-health/faqs/your-first-period) | About 28 days on average, and 21–45 days can also be normal | Guidance for teenagers. For adult cycles, use the adult ranges. | A 35-day cycle fits within all these ranges. It can be typical even though it doesn't match 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 it is outside the NHS and ACOG adult ranges above. These pages don't agree on one shared cut-off. The teenage range also shouldn't be used in place of adult guidance. The table helps you compare the numbers. But the NHS and ACOG explain that other things count too when you decide whether to get help. These include changes from your usual pattern, your bleeding and any other symptoms. ## Bleeding days are only part of the cycle Bleeding can last [between 2 and 7 days, usually about 5](https://www.nhs.uk/conditions/periods/). It is often heaviest in the first 2 days. ACOG's general guidance says a period usually lasts up to 7 days. These are the bleeding days within a cycle. In the April-to-May example, the cycle is still 35 days long, whichever days you bled. How heavily you bleed matters too. ACOG lists heavy bleeding and bleeding between periods among the changes that need checking, even if your period isn't longer than usual. The Office on Women's Health says to get help when pain or bleeding gets in the way of everyday activities. If flow is your main worry, read about [heavy periods and when to get help](https://myruva.co/learn/heavy-periods-how-much-is-too-much). Bleeding that causes problems needs attention, even when your cycle length is what you're used to. ## What changes with age In the first years after periods start, ovulation (the release of an egg) may not happen regularly. Without ovulation, bleeding can be irregular and sometimes heavy. This helps explain why ACOG's [first-period guidance](https://www.acog.org/womens-health/faqs/your-first-period) allows a wider range than its general adult guide. Long gaps in the teenage years still need attention. That guidance says to see a doctor if periods stop for more than three months. Also see a doctor if they come more often than every 21 days, or less often than every 45 days. Pain or bleeding that stops you doing your usual activities also needs checking. Cycles are [usually more regular in your 20s and 30s](https://womenshealth.gov/menstrual-cycle/your-menstrual-cycle). A regular cycle can still vary a little from month to month. It doesn't mean every period has to start on exactly the same schedule. As menopause gets closer, ovulation may become less regular again. Periods can get harder to predict, and bleeding can change in length or heaviness. You can read more about this stage in [perimenopause and period changes](https://myruva.co/learn/perimenopause-and-period-changes). ACOG says to talk about abnormal bleeding, even when some changes around menopause might be expected. ## When your usual pattern shifts Cycle length can [vary from month to month](https://womenshealth.gov/menstrual-cycle/your-menstrual-cycle). If your start date moves by a few days, that tells you what changed. It can't tell you why. Stress, big weight changes and too much exercise are some of the [possible reasons for irregular periods](https://www.nhs.uk/symptoms/irregular-periods/). Hormonal contraception can also change the pattern. For these and medical causes, read [irregular periods: causes and when to see a doctor](https://myruva.co/learn/irregular-periods-causes-and-when-to-see-a-doctor). Working out your cycle length or using calendar estimates can't diagnose the cause. At an appointment, it helps to describe both how long your cycles are and how steady they are. “My cycles are usually near the longer end of the range” means something different from “my periods used to be predictable, and now they've changed”. Either one gives your GP something to work with. You don't have to work out the reason first. ## Getting help for a short, long or changing cycle The [NHS says to see a GP](https://www.nhs.uk/symptoms/irregular-periods/) when your periods are irregular or your normal pattern changes. It counts gaps shorter than 21 days or longer than 35 days as irregular. Book an appointment if: - Your periods keep coming outside that range, or a pattern you knew well changes. - Bleeding lasts longer than 7 days. - You bleed between periods or after sex. - Pain or bleeding gets in the way of work, study or other usual activities. - You have missed three periods in a row. [ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding) and the [Office on Women's Health](https://womenshealth.gov/menstrual-cycle/your-menstrual-cycle) give the bleeding advice above. The NHS says to see a GP after [three missed periods in a row](https://www.nhs.uk/symptoms/missed-or-late-periods/). Ask sooner if a change worries you. If you could be pregnant, take a pregnancy test and follow its instructions. [Most tests can be used from the first day of a missed period](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/). You don't need to wait for several missed periods. ### Pelvic pain that needs urgent or emergency help Pelvic pain with unusual vaginal bleeding needs [urgent assessment](https://www.nhs.uk/symptoms/pelvic-pain/). So does pelvic pain when you might be pregnant. In the UK, contact NHS 111. Elsewhere, use your local urgent medical service. Get emergency care if pelvic pain is severe or getting worse, or if it comes with faintness, difficulty breathing or heavy vaginal bleeding. Call 999 in the UK or your local emergency number, or go to an emergency department (A&E in the UK). ## Quick answers ### Is a 35-day cycle normal? A 35-day cycle falls within the adult ranges given by the NHS, ACOG and the US Office on Women's Health. But a change from your usual pattern, or bleeding that causes problems, still needs attention, even when the length is within a published range. ### Does my cycle have to be exactly 28 days? No. The NHS says the average is around 28 days, and cycles commonly range from 21 to 35 days. The Office on Women's Health uses 24 to 38 days. ### Do I count from the end of my period? No. Count from the first day of one period to the day before the next one starts. The next period starts a new cycle. ## Sources 1. [Periods](https://www.nhs.uk/conditions/periods/), NHS 2. [Abnormal Uterine Bleeding](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding), American College of Obstetricians and Gynecologists 3. [Your menstrual cycle](https://womenshealth.gov/menstrual-cycle/your-menstrual-cycle), Office on Women's Health 4. [Your First Period](https://www.acog.org/womens-health/faqs/your-first-period), American College of Obstetricians and Gynecologists 5. [Irregular periods](https://www.nhs.uk/symptoms/irregular-periods/), NHS 6. [Missed or late periods](https://www.nhs.uk/symptoms/missed-or-late-periods/), NHS 7. [Doing a pregnancy test](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/), NHS 8. [Pelvic pain](https://www.nhs.uk/symptoms/pelvic-pain/), NHS --- This guide is general information to help you keep and use your own records. It isn’t medical advice. If you’re worried about a symptom, or something changes suddenly, talk to a doctor, nurse or pharmacist.