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Free women's health tool · FIGO 2018 · ACOG · Wilcox 1995

Period & Ovulation
Tracker.

Give it three period start dates and it works out when your next three periods are likely to arrive, when your fertile window falls, and whether your cycle sits inside the ranges gynaecologists use.

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Takes about 3 minutes · Nothing leaves your browser · Not a contraceptive method

FIGO range 24–38 days
Wilcox 1995 fertile window
3-month cycle calendar
Red-flag cycle alerts

Enter your cycle dates

Three short steps · Checked against FIGO 2018 and ACOG ranges

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Profile
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Dates
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Details
This tool gives approximate predictions to help you understand your own cycle. It is not a contraceptive method: calendar-based methods carry a 12–24% typical-use pregnancy rate per year (WHO 2018, ACOG). Take reproductive health decisions with a gynaecologist, not with this page.
Age & main goal * Under 18, your cycle is checked against ACOG's adolescent range of 21–45 days. From 18, the FIGO 2018 adult range of 24–38 days applies.
Goal
Diagnosed conditions * Select any that apply. These only change which notes you see at the end: they do not change the calculation.
Hormonal contraception * Hormonal contraception suppresses ovulation, so a fertile window cannot be estimated. If you are on it, that section is withheld rather than guessed at.
NEXT PERIOD PREDICTION
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AVG CYCLE
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days
VARIABILITY
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day range
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About this tracker

Cycle apps usually hand you a date and no working. This one shows its arithmetic. It takes the period start dates you enter, measures the gap between each pair, throws out any that look like logging mistakes, and averages what is left. Your next three periods are that average projected forward, with a band around each date drawn from how much your own cycles actually vary.

Ovulation is estimated backwards from your next predicted period rather than forwards from your last one, because the luteal phase is the steadier half of the cycle: Bull et al. measured a mean of 12.4 days against a follicular phase spanning 10 to 30. The fertile window laid over that is Wilcox's six-day span ending on ovulation day, with his day-specific conception probabilities attached to each day. Your cycle length, variability and bleeding duration are then checked against the FIGO 2018 thresholds, and anything outside them is flagged.

What it cannot do: confirm ovulation, diagnose anything, or work as contraception. Calendar methods have a 12–24% typical-use pregnancy rate per year. If you need to know when you ovulated rather than when you probably will, that takes LH strips, temperature charting or a scan.

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What you get back

📅
Your next three periodsDates plus an honest range around each one
🗓️
Three-month calendarPeriod, fertile and peak days marked out
🌸
Fertile windowGraded day by day using Wilcox's 1995 figures
📊
Cycle health checkYour numbers against the FIGO 2018 thresholds
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Due date estimateNaegele's rule adjusted to your own cycle length

Clinical references

Munro MG et al. FIGO (2018)Two FIGO systems for normal and abnormal uterine bleeding: 2018 revisions. Int J Gynecol Obstet 143(3):393–408
Wilcox AJ et al. (1995)Timing of sexual intercourse in relation to ovulation. NEJM 333(23):1517–21
ACOG (2015)Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. Committee Opinion 651
Bull JR et al. (2019)Real-world menstrual cycle characteristics of more than 600,000 cycles. npj Digit Med 2:83
WHO / JHU (2018)Family Planning: A Global Handbook for Providers, fertility awareness methods

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Common questions

What is a normal menstrual cycle length?

In FIGO's 2018 revision, a normal adult menstrual cycle is 24 to 38 days measured from the first day of one period to the first day of the next. Cycles shorter than 24 days are classified as frequent menstruation and cycles longer than 38 days as infrequent menstruation. For adolescents, ACOG uses a wider range of 21 to 45 days.

How does this tool calculate the fertile window?

It applies Wilcox et al. 1995 in the New England Journal of Medicine, which followed 221 women and found conception only when intercourse fell inside a six-day window ending on the day of ovulation. Day-specific probability rose from about 10% five days before ovulation to about 33% on ovulation day itself. This tool estimates ovulation as 13 days before your next predicted period, then applies those day-specific figures.

Is this period tracker a reliable method of contraception?

No. Calendar-based fertility awareness methods, including this tracker, have typical-use pregnancy rates of 12 to 24% per year according to WHO and ACOG. This tool is for cycle-literacy and educational purposes only and should never be used as a standalone contraceptive method.

When should I see a doctor about my menstrual cycle?

See a gynaecologist if you have a cycle gap longer than 90 days, cycles that are consistently shorter than 24 days or longer than 38 days, or bleeding that lasts more than 8 days. All three fall outside the normal menstrual parameters set out in FIGO's 2018 revision. Sudden pain, bleeding between periods or a clear change from your own usual pattern are also worth a consultation, whatever the numbers say.

How accurate are the period date predictions?

Predictions use the average length of your own recent cycles, so they are only as steady as those cycles are. Bull et al. 2019, analysing 612,613 ovulatory cycles, found the follicular phase varied far more than the luteal phase: mean 16.9 days with a 95% interval of 10 to 30 days. That variation lands directly on your predicted date, which is why this tool shows a range rather than a single day.

How many days after my period do I ovulate?

It depends on your cycle length, not a fixed number of days after your period starts. Ovulation is best estimated backward from your next period: typically about 13 days before it begins. In a 28-day cycle that falls around day 15, but shorter or longer cycles shift the date, which is why this tool calculates it from your own cycle history rather than a fixed day count.

What is a normal luteal phase length?

The luteal phase is the stretch between ovulation and the start of your next period. Bull et al. 2019, analysing 612,613 ovulatory cycles, measured a mean of 12.4 days with a 95% interval of 7 to 17 days. This tool uses 13 days, a mid-range value between that measured mean and the older textbook figure of 14 days.

⚠️ Important: This tracker is for education and cycle literacy. It is not a contraceptive method: calendar-based methods carry a 12–24% typical-use pregnancy rate per year (WHO 2018, ACOG). It does not diagnose any condition. Take decisions about contraception, fertility and reproductive health with a qualified gynaecologist or obstetrician.

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