Free cycle tool

Cycle Phase Calculator

Four names for one continuous month. Enter your last period and your usual cycle length and this will tell you which phase today most likely falls in, what the body is doing in it — and where the edges between them are guesswork rather than measurement.

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

Your cycle, end to end

Three numbers. The first two set where the phases fall; the third only sets where the bleeding stops.

The whole cycle at once

Drag along the bar to look at any day
Showing today The edges are drawn soft because they are soft — no phase begins on a line.
Menstrual Follicular Ovulatory, estimated band Luteal
Today, estimated
Add the first day of your last period

Two numbers and the bar above draws itself. Nothing you type is sent anywhere, and every boundary it draws is an estimate rather than a reading off your body.

Which phase

Fill in the boxes above and this will say what is happening in the phase that day most likely falls in, how far into it you are, and what comes next.

Cycle day
—
counted from the first day of bleeding
Days into this phase
—
of an estimated phase length
Next period, estimated
—
the date every boundary after your period is counted back from

Where the four phases fall on your cycle

Day numbers for a cycle of the length you gave. The ovulatory row is a window rather than a day, and everything after it moves when that window moves. Four is also only one way of cutting a cycle up: the clinical literature as often cuts it in two — follicular and luteal, demarked by ovulation — and those two into six subphases.

    The soft edges on that bar are the honest part of it. Only the first boundary is yours — you told this page when the bleeding started and roughly when it stopped. Everything after it is counted back from a period that has not happened yet. Across 612,613 cycles the average woman's own cycle length moved by about 2.6 days from one month to the next, which is enough on its own to slide every later boundary a couple of days either way, and the ovulatory band moves further still: measured against real cycles, ovulation landed on day 15 in 27% of them, day 16 in 21% and day 14 in 20%. Cycle length can vary by up to 14 days between individuals, and less usually within the same individual.

    This page describes; it does not prescribe. Naming what happens in a phase is physiology. Handing you a diet or a training plan for each one is a product, and the evidence for those schedules does not exist. If you notice you feel different at particular points in your own cycle, that is real and worth writing down — as your own pattern, recorded, rather than as somebody else's timetable.

    Knowing your phase is not contraception, and it is not a diagnosis. The stretch this page calls ovulatory is an estimate a week wide, so nothing on it can be used to avoid a pregnancy — that is a conversation with a doctor, nurse or pharmacist. Nor can a calculator tell you why a cycle felt different this month — if your usual pattern has changed and it worries you, take it to a clinician rather than to a web page.

    How this is worked out

    1. Day one is the first day of bleeding. The menstrual phase runs for as long as your period lasts, and everything after it up to ovulation is the follicular phase — the stretch that does nearly all the moving when a cycle runs long or short.
    2. The ovulatory phase is placed by counting backwards from your next predicted period, because that is the only direction the evidence supports: the NHS puts ovulation at roughly 10 to 16 days before a period rather than at a fixed day. Everything after it is the luteal phase.
    3. The boundaries are drawn as bands, not lines. This is arithmetic on two dates, not a hormone test, and the day it hands you can be out by several days in either direction without anything being wrong.
    Questions

    The short answers

    Probably the one this page names, and "probably" is doing real work in that sentence. A calculator places the phases by counting days from your last period and backwards from your next predicted one. It cannot see hormones, so the ovulatory phase in particular is an estimated band rather than a fact — measured ovulation lands most often on day 15, then 16, then 14, and even inside a tidy 28-day cycle it has been observed across ten different days.
    Menstrual: the days you bleed, which is when the lining shed. Follicular: from the end of your period until ovulation, while follicles in the ovary mature and the lining rebuilds. Ovulatory: the short stretch around the release of an egg. Luteal: from ovulation until the next period. They are names for parts of one continuous process, not four separate states you switch between overnight.
    Because they do. The follicular and ovulatory phases overlap by definition — the hormone rise that ends one is what starts the other — and the boundary between them moves with your own ovulation, which no calendar can see. Across 612,613 cycles, differences in cycle length were caused predominantly by differences in the follicular phase, so the first half stretches and shrinks while the second stays comparatively fixed. A phase edge drawn as a hard line would be the calculator inventing precision.
    This site will not tell you to, because nothing reliable supports the protocols sold under that idea. Describing what happens in a phase is physiology. Prescribing a diet or a training block for each one is a product, and the evidence for it does not exist. If you notice that you feel different at particular points in your own cycle, that is worth recording and worth taking seriously — as your own pattern, not as somebody else's schedule.
    A period usually lasts two to seven days. The follicular phase is the variable one and takes up whatever is left, which is why cycles from 21 to 35 days are all common. The luteal phase is steadier but not fixed: across 612,613 cycles the mean was 12.4 days, with 18% under 11 — not the 14 days the textbook diagrams keep repeating.
    Less usefully, and it will say so on the page. Every phase boundary here is built on an assumed cycle length, so the wider your cycles swing the wider the honest answer gets. If you do not know your own range, work it out first with the cycle length calculator — three or four period start dates is enough to turn a guess into a band.

    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. Nature Reviews Immunology (PubMed Central) (2015). The role of sex hormones in immune protection of the female reproductive tract. https://pmc.ncbi.nlm.nih.gov/articles/PMC4716657/
    4. Journal of Applied Physiology (PubMed Central) (2023). Menstrual cycle hormones and oral contraceptives: a multimethod systems physiology-based review of their impact on key aspects of female physiology. https://pmc.ncbi.nlm.nih.gov/articles/PMC10979803/
    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. Frontiers in Global Women's Health (PubMed Central) (2022). Chronic Stress and Ovulatory Dysfunction: Implications in Times of COVID-19. https://pmc.ncbi.nlm.nih.gov/articles/PMC9168655/
    8. 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 · 7 min read

    The Menstrual Phase, Explained: What Is Actually Happening While You Bleed

    Day one is the first day of full flow, and every number on this site is built on that one date. What the bleeding days are, what commonly comes with them, and where the ordinary ends.

    Read it     Understanding your cycle · 7 min read

    The Follicular Phase: The Half of Your Cycle That Does the Moving

    The first half of the cycle is the part that stretches. What happens in it, why a long cycle is usually a long first half, and what that does to every date a calculator prints.

    Read it     Understanding your cycle · 7 min read

    The Luteal Phase: How Long It Really Lasts, and What Moves It

    The textbook says fourteen days. The measurement says 12.4, with 18% of cycles under 11. What happens in the second half of a cycle, and how to work out your own.

    Read it     Understanding your cycle · 8 min read

    Your Cycle and Exercise: What the Evidence Supports, and What It Does Not

    A meta-analysis of 78 studies found a trivial effect and refused to write guidelines. An umbrella review found no strength difference in 9 of 10 quality studies. Here is what that leaves you with.

    Read it
    Next door

    Ovulation Calculator

    The days ovulation is most likely, worked backwards from your next period — printed as a window, because a single day would be invented.

    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

    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
    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