Period Tracker

What this tool does & how it works

This is a simple, private period tracker. You enter the date your last period started and your usual cycle length, and it predicts your next period by adding that cycle length to the start date. It also shows the period after that and a rough estimated fertile window, calculated around 14 days before the next predicted period — the textbook timing of ovulation in an idealized 28-day cycle.

The key design choice is privacy. Everything you log is stored using your browser's localStorage, which lives on your own device. Your dates are never sent to us or to any server, there is no account, and there is no syncing. The trade-off is that the data is tied to this browser: clearing your browser storage, or using a different device, means starting fresh. The Clear all saved data button erases everything instantly.

How to interpret your results

Treat the predictions as rough signposts. They are most useful for everyday planning — knowing roughly when to expect your period or to pack supplies. The "average 28-day cycle with ovulation on day 14" model is a textbook simplification; real cycles and real ovulation timing vary a great deal between people and from month to month. Logging several cycles will make the tracking more meaningful to you, but it will not make the underlying biology more predictable than it is.

For a sense of how other lifestyle factors interact with your wellbeing, you might also find our sleep calculator and stress check-in useful, since sleep and stress can both influence how you feel across a cycle.

Limitations and when to consult a professional

This tool must never be used as contraception or as a way to plan a pregnancy. Calendar-based predictions are not a reliable fertility-awareness method on their own, and using them to prevent or achieve pregnancy can fail. For contraception or conception, speak to a healthcare provider about methods proven and appropriate for you.

Simple date math also cannot account for the many things that shift a cycle: stress, illness, travel, weight change, hormonal contraception, perimenopause, and conditions such as PCOS or thyroid disorders. If your periods are very irregular, unusually heavy or painful, stop unexpectedly, or change suddenly, please see a healthcare provider — these can be worth investigating. Nothing here is medical advice.

A worked example of the prediction

The math is deliberately simple. If your last period began on the 1st of a month and your typical cycle is 28 days, the tool predicts the next start around the 29th — one cycle length added to the last start date. Give it several past dates and it averages your cycle length rather than assuming 28, which makes the estimate fit your body better. A predicted "fertile window" and ovulation day, where shown, are likewise calendar estimates: ovulation is assumed to fall roughly 14 days before the next expected period, with a window of the preceding days. The key word throughout is estimate — these are projections from your history, not observations of what your body is doing this month.

Why real cycles rarely match the calendar

A "textbook" 28-day cycle is an average, not a norm you should expect to hit. Perfectly healthy cycles commonly range from about 21 to 35 days in adults, and it's normal for length to vary from one month to the next — a difference of a few days between cycles is typical, not a warning sign. Ovulation timing shifts too, which is precisely why calendar prediction can't pinpoint it. The things listed above — stress, illness, travel across time zones, significant weight change, intense training, and the transition years of perimenopause — can all move a cycle, sometimes by a week or more. This ordinary variability is the single biggest reason a date-math tracker should be read as a rough heads-up ("a period is likely around this time"), never as a precise schedule.

Using it well — and its privacy

Where a simple tracker genuinely helps is in spotting your own patterns over time: logging several cycles reveals your personal average length and how much it tends to vary, which is far more informative than any single prediction. That record is also useful to bring to a healthcare provider if something changes. On privacy: everything you log here stays in your device's local storage and is never uploaded, saved to a server, or shared — a deliberate choice for data this personal. Clearing your browser data will erase it, so it lives only where you put it. For the biology behind the phases, see our guide to understanding your cycle. This is an educational tool, not medical advice, and not a method of contraception or conception.

Frequently asked questions