Cycle Tracker
Your personal menstrual cycle companion
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Sign in to unlock Cycle TrackerHow Does the Cycle Tracker Predict My Period?
This tracker learns from your own data instead of assuming a textbook 28-day cycle. Each time you log a period, it records the cycle length (the number of days from one period start to the next) and, if you enter an end date, the period length. To predict your next period it takes a weighted average of your recent cycles — the most recent cycles count more — and adds that to the start date of your last period.
From the predicted next period it works backward to estimate the rest of the cycle: ovulation about 14 days before your next period (the luteal phase is relatively fixed), and a fertile window of 6 days ending on ovulation day. The more cycles you log, the more the predictions reflect your real pattern rather than an average.
Understanding the Phases of Your Cycle
A menstrual cycle is counted from the first day of one period to the first day of the next. A typical cycle is around 28 days, but anything from 21 to 35 days is considered normal, and cycles naturally vary from month to month.
What Is the Fertile Window?
The fertile window is the 6-day span ending on the day of ovulation — the five days before ovulation plus ovulation day itself. This is because sperm can survive in the reproductive tract for up to about 5 days, while the egg lasts only 12–24 hours after it is released. According to ACOG, someone with a 26–32 day cycle is generally most fertile between roughly days 8 and 19. Whether you are trying to conceive or simply want to understand your body, the tracker highlights these days on the calendar.
Worked Example
Suppose your average cycle is 28 days and your last period started on June 1. The tracker adds 28 days to predict your next period on June 29. Ovulation is estimated 14 days before that — about June 15 (roughly two weeks after your period began) — so your fertile window runs the six days ending then, about June 10–15. If your average were 30 days instead, everything shifts later: next period around July 1, ovulation around June 17, fertile window about June 12–17. That is why logging your real cycles matters — a two-day change in cycle length moves your fertile days too.
Regular vs. Irregular Cycles
The tracker also scores how regular your cycles are, based on how much their length varies (standard deviation):
Some month-to-month variation is completely normal. Stress, illness, travel, intense exercise, breastfeeding, and conditions such as PCOS or thyroid issues can all change cycle length. Tracking helps you and your doctor see your real pattern over time.
When Should You See a Doctor?
A cycle tracker is for personal awareness, not diagnosis. Consider talking to a healthcare provider if your cycles are consistently shorter than 21 days or longer than 35 days, vary widely from month to month, suddenly change or stop, become very heavy or painful, or if you have been trying to conceive without success. The history you log here is useful information to bring to that appointment.
Frequently Asked Questions
How accurate are cycle predictions?
Predictions improve with more data. With no logged cycles, the tracker uses your default cycle length; after 3 or more logged cycles it weights your most recent cycles to personalize predictions to your own pattern. No predictor is exact, because cycles naturally vary from month to month.
What is the fertile window?
The fertile window is the 6-day span ending on the day of ovulation — the 5 days before ovulation plus ovulation day itself. Sperm can survive up to about 5 days, while the egg lasts roughly 12–24 hours, so conception is most likely in these 6 days. This tracker estimates ovulation about 14 days before your next predicted period.
How do I track irregular cycles?
Just log each period as it happens. The tracker adapts to your actual data instead of assuming a fixed 28-day cycle, and it shows a regularity score based on how much your cycle length varies. Predictions for very irregular cycles are less precise — log consistently for the best estimate.
How many cycles should I track before predictions are reliable?
You get predictions right away, but accuracy rises with data: 1–2 cycles give low confidence, 3–5 give medium confidence, and 6 or more give high confidence. The tracker labels the confidence level so you know how much weight to give each prediction.
Can I use a period tracker as birth control?
No. Calendar-based predictions are estimates and should not be relied on to prevent pregnancy. Ovulation can shift with stress, illness, or travel, and the fertile window can fall outside the predicted days. Use a medically recommended contraceptive method if you are avoiding pregnancy.
When should I see a doctor about my cycle?
Talk to a healthcare provider if your cycles are consistently shorter than 21 days or longer than 35 days, vary widely from month to month, suddenly change, stop, become very heavy or painful, or if you have been trying to conceive without success. Tracking your cycle gives your provider useful information.
Related Tools
- Ovulation Calculator — estimate your fertile window and ovulation day from a single cycle, no sign-in needed
- Due Date Calculator — if you conceive, find your estimated due date from your last period
- Pregnancy Weight Tracker — follow healthy weight gain by week once you are expecting
- Baby Name Generator — start dreaming up names while you plan
Methodology and Sources
Predictions use a recency-weighted average of your logged cycle lengths. Ovulation is estimated as 14 days before the next predicted period (a relatively fixed luteal phase), and the fertile window is the 6-day interval ending on ovulation day, consistent with the bodies below. These are population-based estimates; your own ovulation timing can vary, and ovulation tests or clinical monitoring are more precise.
- ACOG — Fertility Awareness-Based Methods & the Fertile Window
- NICHD — About Menstruation & the Menstrual Cycle
- ACOG — Trying to Get Pregnant? Here’s When to Have Sex
- Johns Hopkins Medicine — Calculating Your Monthly Fertility Window
Note: This tracker is for general information and is not medical advice or a contraceptive method. Predicted periods, fertile windows, and ovulation dates are estimates that can shift with stress, illness, and lifestyle. Talk to a healthcare provider about contraception, fertility, or any concerns about your menstrual health.