Period Tracker
Private cycle tracker with predictions and calendar.
About this tool
Log the first (and optionally last) day of each period to see your current cycle day, average cycle and period length, the next three predicted periods and an estimated fertile window on a month calendar.
How cycle predictions work
A menstrual cycle is counted from the first day of one period to the first day of the next. Typical adult cycles run 21 to 35 days and vary by a few days from month to month. This tracker averages your last six cycle lengths (ignoring gaps that look like a missed log) and adds that to your last period to predict the next three. Ovulation usually happens about 14 days before the next period, because the phase after ovulation is fairly constant, so the tracker estimates ovulation that way and marks the five days before it plus the day after as the fertile window. The more cycles you log, the better the averages; if recent cycles differ by more than about a week, predictions become less reliable and the tracker says so. Your data stays in this browser: no account, nothing sent anywhere, with export and import for backups. Important: predictions are estimates from averages, not a method of contraception, and can't confirm or rule out pregnancy; this is not medical advice. Very painful, very heavy, missed or irregular periods, or cycles often under 21 or over 35 days, are worth discussing with a doctor. The ovulation calculator and the due date calculator are related.
Frequently asked questions
How is the next period predicted?
Your average cycle length from up to six recent cycles is added to the start of your last period.
What is a normal cycle length?
For adults, about 21 to 35 days is typical, with some variation month to month.
Can I use this as contraception?
No. Fertile window estimates are averages and are not reliable enough to prevent pregnancy.
Is my period data private?
Yes. It is saved only in this browser, never sent anywhere, and you can export or delete it any time.
Where is my data stored?
Only in this browser on this device. There is no account, nothing is sent anywhere, and Clear all deletes it; export a copy before clearing browser data or changing device.