Emmenia
    Get started

    Free tool · no sign-up

    Cycle length check: how regular are your periods?

    Enter the first day of your last few periods. You'll get your average cycle length, your shortest and longest, and a plain-language read on how steady your pattern is — the same numbers a clinician asks for.

    Add the first day of each of your recent periods, oldest or newest first — order does not matter.

    Add at least three period start dates to see your pattern.

    The number that actually matters

    Most people know their average cycle length. Far fewer know their variation — the gap between their shortest and longest cycle. That single number decides how much you can trust a predicted date, and it is the one clinicians reach for when deciding whether a pattern needs looking into.

    Reading your result

    • Under 5 days of variation: a steady pattern. Predictions should land within a day either side.
    • 5 to 9 days: normal month-to-month movement. Plan with a window rather than a fixed date.
    • Over 9 days: a wide swing. Often explained by stress, illness or a life-stage change, and worth mentioning at a routine appointment.

    Variation is not automatically a problem. It just means calendar maths alone will not serve you well — you need a forecast that learns from your own history and widens its window honestly when your cycles move.

    Common questions

    What counts as an irregular cycle?
    Clinically, cycles are usually described as irregular when the gap between the shortest and longest cycle in a year is more than about 9 days, or when cycles fall outside the 21-to-35-day range. A one-off odd cycle after illness, travel or high stress is common and not the same thing.
    How many cycles do I need to log?
    Three start dates give you two cycle lengths — enough for a rough read. Six start dates over about half a year give a much fairer picture, because it captures the normal month-to-month wobble.
    What makes cycles become irregular?
    Stress, big changes in weight or training load, disrupted sleep, illness, some medications, thyroid conditions, PCOS and the transition towards menopause can all shift cycle length. Persistent change is the signal worth investigating, not a single unusual month.
    When should I talk to a clinician?
    If cycles are consistently shorter than 21 days or longer than 35, if you bleed between periods or after sex, if bleeding is unusually heavy, or if your pattern changes suddenly and stays changed. Bring these numbers with you — they make the conversation much faster.

    Want this worked out for you, every cycle?

    Emmenia learns your real cycle from what you log, predicts cramps before they start and builds a daily relief plan. Free to start.

    Create a free account

    More free tools

    Period calculator

    Predict your next three period start dates.

    Ovulation calculator

    Estimate your fertile window and ovulation day.

    These calculators give estimates for general wellness planning. They are not contraception, not a pregnancy test and not medical advice. Speak to a clinician about pain, bleeding or cycle changes that worry you.