Health tool
Period tracker and prediction
Enter the first day of each period. With two or more records, the tool estimates your cycle length and predicts the next start date and fertile window.
Add a period
Recent cycle lengths
Each bar is days between one period start and the next; the line is the 21–35 day reference range.
Recorded periods
How the prediction works
The tool measures the days between your logged period starts, drops clearly abnormal gaps (under 21 or over 45 days — often a missed record), and takes the median as your cycle length. The next period is predicted as last start plus that length, with a range from how much your cycles vary.
The fertile window is estimated as roughly 14 days before the predicted next period, lasting about 5–6 days — the same convention most open-source trackers use. Real ovulation shifts, so treat it as an estimate.
Frequently asked questions
Where is my data stored?
Only in this device's browser storage — nothing is uploaded. This data is private, so be mindful of shared browsers and devices.
How does the prediction work?
It takes the gaps between your recent period start dates, drops obvious outliers, and uses the median as your cycle length. The fertile window is estimated as roughly 14 days before the predicted next period.
What is a normal cycle length?
Most adult cycles run 21–35 days with bleeding for 2–7 days. Shifting within a week is common; cycles consistently under 21 or over 35 days, or suddenly irregular, are worth a doctor's visit.
Can I use the prediction for contraception?
No. Ovulation shifts with stress, illness, and schedule changes, so calendar-based “safe days” are unreliable. Use a proper contraceptive method if you need one.
When is a late period a concern?
If your cycle is usually regular and this one is over a week late, take a pregnancy test first if relevant. Still irregular after that, or with unusual flow or worsening pain, see a doctor.