Free cycle tool

Cycle Length Calculator

Your cycle length is the number of days from one period's first day to the next period's first day. The NHS puts the usual adult range at 21 to 35 days, and ACOG at 24 to 38. What matters more than the average is the swing: log four start dates and this shows your shortest, your longest and the gap between them.

Add the first day of each of your last few periods. This works out your average cycle, your shortest and longest, and how far your cycles swing — and explains, plainly, when that swing is ordinary and when it is worth a conversation.

Free, and no account.

The first day of each period

Newest or oldest first, it does not matter — they are sorted before anything is counted. Four dates give you three cycles.

    Your numbers

    Add at least two dates and this fills in.

    Average cycle
    —
    days, start to start
    Shortest
    —
    days
    Longest
    —
    days
    Your swing
    —
    days between shortest and longest

    Every cycle, on one axis

    days
    153045
    21 to 35 days — the NHS's usual band your average one cycle
    What the numbers say
    Add a few dates above

    Two period start dates give one cycle. Three give a hint. Six give something you can plan around.

    A number here is not a verdict on you. This page counts the gaps between dates you typed and compares them with the bands the NHS and ACOG publish. It cannot examine you, it does not know your age, your history or anything else about you, and it cannot diagnose anything — no web page can.

    What the bands actually are. The NHS says most people have a period around every 28 days and that every 21 to 35 days is common, and calls periods irregular when the gap is under 21 days or over 35. ACOG counts 21 to 45 days as normal for teenagers, because it can take six years or more after periods start for a cycle to settle. Falling outside a band is a reason to ask someone, not a reason to worry on your own.

    When to speak to someone. The NHS suggests seeing a GP if your periods are irregular or your normal pattern changes, if you have missed three periods in a row, or if your periods have not started by the time you are 15. Take these numbers with you — an average, a range and the number of cycles they came from is exactly what makes that appointment quick.

    How this is worked out

    1. Each cycle length is the number of days from one period start to the next period start. Four start dates give three cycles; the more you add, the steadier the average.
    2. The average is the plain mean of those cycles. The range is the shortest and the longest. The variation figure is the gap between them — the number that actually tells you how predictable your cycle is.
    3. Nothing here is a diagnosis. The thresholds quoted are the ones the NHS and ACOG publish for when to speak to a clinician, and they exist to start a conversation, not to end one.
    Questions

    The short answers

    Count from the first day of one period to the day before the next period starts. If you bled on the 3rd of March and again on the 31st of March, that cycle was 28 days. Do it for several months and you have a range rather than a single number, which is far more useful.
    There is a band, not a number. The NHS says most people have a period around every 28 days, and that anywhere from every 21 to every 35 days is common. Teenagers get a wider band — ACOG counts 21 to 45 days as normal — and FIGO defines the adult range as 24 to 38. Meanwhile, across 612,613 tracked cycles, the average length was 29.3 days and only 13% were the textbook 28. Your own normal is the band your own cycles fall in.
    Some movement is expected: in that same 612,613-cycle dataset, the average person's own cycle length moved by about 2.6 days between months. The NHS calls periods irregular when the gap between them is under 21 days or over 35, notes this is most likely when periods first start and when nearing the menopause, and suggests speaking to a GP if your periods are irregular or your normal pattern changes.
    Because nearly all of the variation happens before ovulation, not after. The researchers behind the 612,613-cycle study found that differences in cycle length were caused predominantly by differences in the follicular phase — the stretch from your period to ovulation, which stress, illness, travel and big changes in sleep, weight or exercise can all move. The stretch from ovulation to your next period is steadier. That is why a late period usually means late ovulation rather than a longer second half.
    Three cycles gives you a rough idea, six gives you something you can plan around, and twelve tells you what your year looks like. This page will happily work from two, and it will tell you when it is working from too little to say much.
    It can be, and it very often is not. Irregular cycles have many ordinary explanations and some medical ones, and no calculator can tell which is which — that needs a clinician who knows your history. What this page can do is give you the clean numbers to bring to that appointment: your average, your range, and how many cycles it is based on. The NHS says to see a GP if you have missed three periods in a row, or if your periods have not started by the time you are 15.

    Where this comes from

    1. NHS (2025). Periods. https://www.nhs.uk/conditions/periods/
    2. NHS (2025). Irregular periods. https://www.nhs.uk/symptoms/irregular-periods/
    3. NHS (2026). Missed or late periods. https://www.nhs.uk/symptoms/missed-or-late-periods/
    4. American College of Obstetricians and Gynecologists (2015). Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2015/12/menstruation-in-girls-and-adolescents-using-the-menstrual-cycle-as-a-vital-sign
    5. Human Reproduction (PubMed Central) (2022). The FIGO systems for nomenclature and classification of causes of abnormal uterine bleeding. https://pmc.ncbi.nlm.nih.gov/articles/PMC9527465/
    6. npj Digital Medicine (PubMed Central) (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/
    7. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH) (2017). Menstruation: Condition Information. https://www.nichd.nih.gov/health/topics/menstruation/conditioninfo

    Every link above was checked when this page was last updated. Athena is not affiliated with any of these organisations, and none of them has reviewed this page. Nothing here is medical advice.

    Go deeper

    The reading behind this page

    Understanding your cycle · 6 min read

    Cycle Length: What Is Normal, and What the Numbers Actually Say

    Four respectable bodies, four different answers, all on one chart. Then the numbers from 612,613 real cycles, and what to do with your own.

    Read it     Understanding your cycle · 7 min read

    Irregular Periods: When Normal Variation Is Worth a Conversation

    Almost everybody thinks their periods are irregular, and most are describing ordinary movement. Here is what the word means to the NHS, to ACOG and to NICHD, and where each of them draws the line.

    Read it     First periods and practical life · 8 min read

    Tracking Your First Year of Periods: What to Write Down and What to Ignore

    A messy first year is the expected result rather than a fault. Here is the one thing worth writing down, the things that are not, and the two thresholds that are worth an appointment.

    Read it     Privacy and your data · 9 min read

    The Most Private Period Tracker: What Actually Protects You

    Most privacy claims are promises. A few are architecture. This is how to tell which one you are being offered — and the seven questions that separate them.

    Read it
    Next door

    Period Calculator

    Your last period plus your usual cycle length, six cycles laid out on the moon ring, and the whole thing as a printed calendar or a calendar file — windows, never single days.

    Open it

    Cycle Phase Calculator

    Which phase you are probably in today, what is actually happening in it, and a straight answer about how blurred the edges between them are.

    Open it

    Period Late Calculator

    How late is late, measured against your cycle rather than a textbook one — plus the ordinary explanations, and the day a test would actually mean something.

    Open it
    The app this site is for

    Athena keeps all of this on your phone.

    A calculator answers the one question you brought it. Athena is the same arithmetic living on your device — the moon ring, an honest calendar with ranges instead of false precision, and a database that never leaves the phone because there is no account and no server behind it.

    On the App Store and Google Play, for iPhone and Android.

    What Athena is