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    Period calculator: when is my next period due?

    Enter the first day of your last period and your usual cycle length. You'll get your next three predicted start dates, your fertile window and the phase you're in today — worked out in your browser, with nothing sent to us.

    First day of one period to the day before the next. 28 if you are unsure.

    Your next period should start

    Sat, 19 September 2026

    In 28 days · through 23 Sept

    Estimated ovulation
    Sat, 5 September 2026
    Fertile window
    31 Aug – 6 Sept
    Today
    Cycle day 1 · Menstrual phase

    Bleeding days. Energy is usually lowest — heat, gentle movement and rest tend to help most here.

    Plan the next three

    • Sat, 17 October 2026
    • Sat, 14 November 2026
    • Sat, 12 December 2026

    Emmenia

    My next period is due

    Sat, 19 September 2026

    Days away
    28
    Cycle length
    28 days
    Fertile window
    31 Aug – 6 Sept
    Ovulation (est.)
    5 Sept

    Estimate for wellness planning — not contraception or medical advice.

    The card only contains the dates you entered here — no account details, and nothing is uploaded.

    How the calculation works

    A menstrual cycle is measured from the first day of bleeding to the day before your next period begins. To predict your next start date, this calculator adds your typical cycle length to that first day, then repeats it to project the two cycles after that.

    The luteal phase — the stretch between ovulation and your period — is the most consistent part of the cycle for most people, at roughly 14 days. That is why the fertile window here is anchored backwards from your predicted period rather than forwards from your last one: it stays more accurate when your cycle length shifts.

    What a normal cycle length looks like

    Adult cycles commonly run between 21 and 35 days, and it is normal for the same person to vary by a few days from cycle to cycle. Cycles are typically longer and less predictable in the first years after periods begin and again in the run-up to menopause. Persistent cycles shorter than 21 days or longer than 35, bleeding between periods, or a sudden change in your pattern are all worth raising with a clinician.

    Getting ahead of period pain

    Knowing the date matters most because cramps are far easier to blunt before they peak. If your period is due Thursday, starting heat, hydration and gentle movement on Wednesday evening tends to work better than reacting on the day. That is the whole idea behind Emmenia: predict the day, then pre-load the relief.

    Common questions

    How do you calculate when my next period will start?
    We add your usual cycle length to the first day of your last period. A cycle is counted from the first day of one period to the day before the next one starts, so a 28-day cycle starting on the 1st predicts the next start on the 29th.
    What if I do not know my cycle length?
    Leave it at 28 days, which is the most common average, and the estimate will still be in the right region. Once you have logged two or three real cycles the number becomes personal to you and the prediction tightens considerably.
    How accurate is a period calculator?
    For steady cycles, a simple calculation is usually right within a day or two. If your cycles vary by more than about a week, treat the date as the middle of a window rather than a fixed day — apps that learn from your logged history handle that variation far better.
    Can I use this as contraception?
    No. Ovulation timing shifts with stress, illness, travel and life stage, and sperm can survive several days. Use a medically approved contraceptive method and speak to a clinician about what suits you.
    Do I need an account to use this?
    No. The calculator runs entirely in your browser, nothing is sent to us, and there is no sign-up. An account only helps if you want the dates learned from your real history and paired with relief routines.

    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

    Ovulation calculator

    Estimate your fertile window and ovulation day.

    Cycle regularity check

    See how much your cycles actually vary.

    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.