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.

Frequently asked questions