Free cycle tool

Period Calculator

Enter the first day of your last period and how long your cycle usually runs. Athena estimates your next three periods and the window around ovulation — and shows you how wide that estimate really is.

Runs entirely in your browser. No account, no email, and nothing you type is sent anywhere.

Your last period and your usual cycle
How much your cycles move

This is what turns a date into a range. Leave them alone and the calculator assumes three days either side, because almost nobody's cycle is the same length twice running.

Count the days between two dates

The gap between two period start dates is that month's cycle length — the number every other prediction on this site is built from. It also counts any two dates you like.

Your cycle drawn as a ring, with the estimated period days and fertile window marked.
—
cycle day
Period Pregnancy possible Ovulation likely Rest of the cycle
Your next period, estimated
Add your last period above

Two numbers and this page fills itself in. Nothing is sent anywhere.

The next three periods

Each one is the middle of a window. The window widens the further ahead you look, because every cycle in between can move.

    The fertile window, honestly

    Worked backwards from your predicted period rather than forwards from your last one — which is the only direction the evidence supports.

    Ovulation likely between
    —
    roughly 10 to 16 days before your period
    Pregnancy possible
    —
    the NHS puts sperm survival at up to 7 days, so the window opens well before ovulation
    —
    days between those dates

    Put a date in both boxes above.

    Read this before you trust a date on this page. Everything above is arithmetic on the numbers you typed. It cannot see your body. Real cycles move by a few days in either direction routinely, and a stressful month, an illness, travel or a change in sleep can move them further.

    This is not contraception, and it cannot be made into contraception. Ovulation shifts, sperm can survive in the body for up to seven days, and no calculator can see either. Fertility awareness followed imperfectly is about 76% effective — 24 women in 100 get pregnant in a year — and the NHS does not officially recommend any app or device for it. If you are trying to avoid or plan a pregnancy, talk to a doctor, nurse or pharmacist.

    It also cannot tell you whether you are pregnant, and it cannot diagnose anything. If your periods stop, if the gap between them keeps changing and that is new for you, or if something else about them worries you, that is a conversation with a clinician rather than a calculator.

    How this is worked out

    1. A cycle is counted from the first day of one period to the day before the next. Your next period is estimated by adding your cycle length to the first day of your last one.
    2. The fertile-window estimate works backwards from that predicted date rather than forwards from your last period. The NHS puts ovulation at roughly 10 to 16 days before the next period, so the window is drawn from the predicted date and never from "day 14".
    3. The range shown around every predicted date is your own: it comes from the shortest and longest cycle you enter. If you give a single cycle length, the calculator assumes a few days either side, because almost nobody's cycle is the same length twice running.
    Questions

    The short answers

    It is arithmetic, not a forecast. A calculator adds your cycle length to your last period and shows you the result. Real cycles move by a few days in either direction all the time — illness, travel, stress, a bad run of sleep and plenty of ordinary things shift them — so treat the date as the middle of a range rather than a promise. That is why this page always prints the range as well as the date.
    Switch the calculator to "From date to date". Put in the first day of one period and the first day of the next, and it counts the days between them. That number is your cycle length for that month, which is the figure every other prediction on this site is built from.
    That is normal, and the calculator is built for it. Enter your shortest and longest cycle and it will show a window instead of a single day. In a study of 612,613 cycles, the average person's own cycle length moved by about 2.6 days from one month to the next. The NHS suggests speaking to a GP if your periods are irregular or your usual pattern changes.
    No, and this is the one answer on the page with no hedging in it. Sperm can survive in the body for up to seven days, ovulation moves around, and a calculator cannot see either. Fertility awareness followed imperfectly is about 76% effective, meaning 24 women in 100 get pregnant in a year — and the NHS does not officially recommend any app or device for this. If you are trying to avoid or plan a pregnancy, talk to a doctor, nurse or pharmacist.
    Only in this browser, and only if you want. Your dates are kept in your own browser's local storage so the page remembers them next time, and there is a button to clear them. Nothing is sent to a server: there is no account here, and no request carries your dates anywhere.
    Because nothing on a calendar knows that. When researchers checked real ovulation against real cycles, day 15 was the commonest day at 27%, then day 16 at 21%, then day 14 at 20% — and even within a tidy 28-day cycle the observed ovulation days spread across ten of them. Printing one day would be a made-up number dressed as a measurement, so this page prints the window and says how wide it is.

    Where this comes from

    1. NHS (2025). Periods. https://www.nhs.uk/conditions/periods/
    2. NHS (2025). Periods and fertility in the menstrual cycle. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
    3. NHS (2025). Irregular periods. https://www.nhs.uk/symptoms/irregular-periods/
    4. NHS (2025). Natural family planning (fertility awareness). https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/
    5. 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/
    6. Human Reproduction Open (PubMed Central) (2020). Real-life insights on menstrual cycles and ovulation using big data. https://pmc.ncbi.nlm.nih.gov/articles/PMC7164578/
    7. American College of Obstetricians and Gynecologists (2023). Fertility Awareness-Based Methods of Family Planning. https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning

    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 · 8 min read

    When Will I Get My Period? What a Quiz Can and Can't Tell You

    Every quiz that promises a date is guessing. Here is what the signs actually mean, what the timelines usually look like, and an estimator that gives you a range and admits it.

    Read it     Understanding your cycle · 7 min read

    The Moon and Your Menstrual Cycle: What the Research Actually Found

    Two rhythms of about twenty-nine days, and a very old story about why. Here is what the studies actually found — including the ones that found nothing.

    Read it
    Next door

    Cycle Length Calculator

    Log a few period start dates and read your own numbers: average, shortest, longest, and how much the whole thing moves.

    Open it

    Luteal Phase Calculator

    The days from ovulation to your next period — counted exactly when you know the date, and estimated with the uncertainty shown when you do not.

    Open it

    Printable Period Tracker

    Build a paper tracker for any month or a whole year, then print it. Flow key, symptoms, an optional temperature row — and a gentler version for a first tracker.

    Open it
    The app this site is for

    Athena keeps all of this on your phone.

    The calculators here forget you the moment you close the tab. 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.

    In development for iPhone and Android. Not released yet — when it is, it will be on this site.

    What Athena is