Know your cycle
Cycle Regularity Checker
Enter your last three to six cycle lengths — or just your period start dates — and see your average, your shortest-to-longest range, and whether the variation counts as regular under the FIGO criteria clinicians use. Everything runs on your device; nothing is sent or saved.
A cycle or two thrown off by illness or stress is completely normal — this reads your pattern across several cycles, and it's an educational read, not a diagnosis.
Cycle regularity analyzer
Based on the FIGO (2018) normal-cycle parameters. This is general information, not a diagnosis — one or two irregular cycles after illness or stress is normal, and only a clinician can work out the cause of a persistent pattern.
How it works
Your cycle is a vital sign — here's how clinicians read it
ACOG calls the menstrual cycle a vital sign: like pulse or blood pressure, its pattern says something about overall health. The international yardstick is FIGO's 2018 classification, which separates two things people often blur. Frequency is how far apart your periods are — anywhere from 24 to 38 days is normal, so a 35-day cycle is not "late," it's just yours. Regularity is how much that number moves cycle to cycle: the gap between your shortest and longest recent cycles. A variation of up to 7–9 days (9 at ages 18–25, 7 at 26–41, 9 at 42–45) still counts as regular — a perfectly repeating 28-day cycle is a textbook fiction, not a requirement.
This tool does that same arithmetic on the cycles you enter: average, shortest, longest, and the shortest-to-longest spread, read against the FIGO bounds. The spread also matters for timing: ovulation happens about 14 days before the next period (the second half of the cycle is the steady one), so the wider your range, the wider the window your ovulation day can land in — and the less useful pure calendar math becomes for predicting fertile days.
What it can't do is name a cause. Variation beyond the bounds is common and usually explainable — PCOS, thyroid or prolactin issues, stress, weight change, perimenopause — but sorting that out takes a history and a few blood tests, not a calculator. Treat the result as a well-grounded starting point for a conversation with your provider.
FIGO normal menstrual cycle parameters (2018)
| Parameter | Normal | Outside normal |
|---|---|---|
| Frequency | 24–38 days between period start days | Under 24 days = frequent · over 38 days = infrequent |
| Regularity, ages 18–25 | Shortest-to-longest variation of 9 days or less | More than 9 days = irregular |
| Regularity, ages 26–41 | Shortest-to-longest variation of 7 days or less | More than 7 days = irregular |
| Regularity, ages 42–45 | Shortest-to-longest variation of 9 days or less | More than 9 days = irregular |
| Duration of bleeding | 8 days or less | More than 8 days = prolonged |
Source: FIGO AUB System 1, 2018 revisions (Munro et al.). The bounds are defined for ages 18–45; wider variation is common — and often normal — in the first years after a first period and in the years approaching menopause.
Next steps
Related tools
Ovulation calculator
Turn your cycle length into a fertile window and your most fertile days.
When to see a specialist
How long to try before an evaluation — irregular cycles can move that timeline up.
Implantation calculator
When a fertilized egg implants after ovulation — the steady half of the cycle at work.
All pregnancy tools
The full suite — from timing conception to dating a pregnancy.
Sources
Where this comes from
The 24–38-day frequency range and the ≤7–9-day regularity bounds are FIGO's, from the 2018 revision of its AUB System 1 parameters (Munro et al.). The when-to-see-a-clinician thresholds — persistent variation beyond the bounds, cycles under 24 or over 38 days, 90+ days without a period, bleeding between periods, and the 6-to-12-month trying-to-conceive timelines — follow ACOG's irregular-bleeding and infertility-evaluation guidance. We apply the published criteria; we don't invent our own.
Medically Reviewed & Fact-Checked · Updated
Reviewed by EasySTD Editorial Team
Compiled and checked by EasySTD's editorial team against CDC and public-health sources. This is educational information, not a substitute for advice from a licensed clinician. Our editorial guidelines →
4 Sources
Data & references
- Munro MG, et al. The two FIGO systems for normal and abnormal uterine bleeding symptoms: 2018 revisions. Int J Gynaecol Obstet, 2018https://pubmed.ncbi.nlm.nih.gov/30198563/
- ACOG: Abnormal Uterine Bleeding (patient FAQ)https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding
- ACOG Committee Opinion 651: Menstruation in girls and adolescents — using the menstrual cycle as a vital signhttps://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2015/12/menstruation-in-girls-and-adolescents-using-the-menstrual-cycle-as-a-vital-sign
- ACOG: Evaluating Infertility (when to seek an evaluation)https://www.acog.org/womens-health/faqs/evaluating-infertility
EasySTD is an information and comparison directory, not a healthcare provider. This tool is general information based on published FIGO and ACOG guidance: it is not medical advice or a diagnosis, and it does not create a doctor-patient relationship. A persistent pattern of irregular cycles deserves a clinician's evaluation — this tool helps you describe it, not replace it.
Good to Know
Cycle regularity: frequently asked questions
What regular actually means, how to count a cycle, when a couple of odd cycles are fine, what causes irregularity, and when to get checked.
What counts as a "regular" menstrual cycle?
The FIGO classification (the 2018 revision of its AUB System 1, used by gynecologists worldwide) defines a normal cycle frequency as 24 to 38 days, counted from the first day of one period to the first day of the next. Regularity is a separate measure: how much your shortest and longest cycles differ over recent months. Variation of up to 7–9 days counts as regular — up to 9 days at ages 18–25, up to 7 days at 26–41, and up to 9 days again at 42–45. So cycles that swing between 27 and 33 days are regular; a textbook 28-day cycle is not required, and very few people have one.
How do I work out my cycle length?
Count from the first day of full bleeding in one period to the day before the next period starts — in practice, the number of days between one period's start date and the next period's start date. Light spotting before your flow doesn't count as day 1. Most period-tracking apps compute this automatically, but a plain calendar works just as well: note each start date and count the gaps. Three cycles is the minimum for a meaningful read; six or more gives a much steadier picture of your true average and range, which is why this tool accepts up to six.
A couple of my cycles were off — is that a problem?
Usually not. Illness, fever, significant stress, travel across time zones, big changes in sleep or exercise, rapid weight change, and stopping hormonal birth control can each delay ovulation and stretch — or occasionally shorten — a cycle or two. That's expected biology, not a disorder. The FIGO regularity criteria describe your pattern over many months, not any single cycle, so one or two outliers within an otherwise steady pattern are normal. What deserves a closer look is variation that persists cycle after cycle, or periods that stop arriving altogether.
What causes persistently irregular cycles?
The most common cause is polycystic ovary syndrome (PCOS), which affects roughly one in ten people with ovaries. Other frequent, findable causes include thyroid over- or under-activity, elevated prolactin, significant ongoing stress, substantial weight loss or gain (including intense training with low energy intake), and perimenopause — the years approaching menopause, when cycles naturally scatter. Some medications and breastfeeding also suppress or shift ovulation. The encouraging part: most of these are identified with a history, an exam, and a few blood tests, and most are treatable or manageable once named.
When should I see a clinician about my cycles?
Book a visit if your shortest-to-longest variation stays beyond the FIGO bound for your age (more than 7–9 days, month after month), if your cycles run shorter than 24 days or longer than 38 days, if you go 90 days or more without a period and aren't pregnant, or if you bleed between periods or after sex. If you're trying to conceive, irregular cycles matter sooner: the standard advice is an evaluation after 12 months of trying — or after 6 months if you're 35 or older — and persistently irregular cycles are a reason to go earlier still, since they often signal irregular or absent ovulation.
Is what I enter private?
Yes. This tool runs entirely in your browser. The cycle lengths or dates you enter are analyzed on your own device and are never sent to a server, saved, or shared — close the tab and they're gone. We treat menstrual-cycle data as sensitive by default in a post-Dobbs world, which is exactly why nothing here is stored or transmitted. The result is general educational information based on published FIGO criteria; it isn't medical advice or a diagnosis, and it can't replace a clinician's read of your history.
Have a different question? Browse the full pregnancy FAQ library, every question we answer, in one place.