Period Tracker Offline 100% Private

Log your menstrual cycle, predict your next period and ovulation day, and export your data as CSV — all stored only in your browser. No sign-up. No account. No data sent anywhere.

Period start
Period end / mid
Est. ovulation
Predicted period

Add / edit cycle entry

Tap a day on the calendar to fill the start date, or type it manually. End date is optional.

How it works

Every piece of data you enter stays only on your device, saved in your browser's localStorage. Nothing is transmitted to any server. You can use this tool on airplane mode.

Cycle length Days from the start of one period to the start of the next. A typical cycle is 21–35 days.
Next period prediction Last period start date + your average cycle length (mean of all logged cycles).
Ovulation estimate Predicted next period start − 14 days (standard luteal phase). Fertile window is roughly 5 days before ovulation.
Luteal phase The phase after ovulation until period starts. Usually 12–16 days, assumed 14 here. Cycle length varies; luteal phase is relatively constant.

Example: You logged periods starting Jan 1, Jan 29, Feb 25 → cycle lengths 28 and 27 days → average 27.5 days → next period predicted Mar 24 → ovulation estimated Mar 10.

Frequently asked questions

Is my period data private? Can anyone see it?
Your data is stored exclusively in your browser's localStorage — the same place a website might remember your preferences. It never leaves your device. No account, no server, no analytics. If you clear your browser data or use a private/incognito window, the data will be gone. Use the CSV export regularly to keep a backup.
How accurate is the next period prediction?
The prediction uses your own average cycle length calculated from all logged entries. The more cycles you log, the more accurate the prediction becomes. Individual cycles can vary by several days due to stress, illness, or hormonal changes, so treat the predicted date as an estimate ± 2–4 days. This tool is not a medical device and does not replace advice from a healthcare provider.
How is ovulation day estimated?
Ovulation is estimated as your predicted next period start date minus 14 days, which represents the assumed luteal phase length. The luteal phase (from ovulation to period) is relatively consistent at 12–16 days for most people, so subtracting 14 days gives a reasonable midpoint estimate. Your fertile window is approximately 5 days before ovulation through ovulation day itself.
What does "cycle length" mean, and what is normal?
Cycle length is the number of days from the first day of one period to the first day of the next. A typical cycle is 21–35 days, with 28 days being the often-cited average. Cycles shorter than 21 days or longer than 35 days may be worth discussing with a doctor, though significant variation is common and often normal.
Can I export my data and use it elsewhere?
Yes. Click "Export CSV" in the History tab to download a spreadsheet-compatible file with all your period entries and any sleep, fasting, and weight logs. You can open it in Excel, Google Sheets, or share it with a healthcare provider. The CSV includes start date, end date, cycle length, and any optional daily metrics.
What happens if I skip logging a period?
The tracker calculates cycle lengths only between consecutive logged entries. Gaps in your log will produce longer-than-normal apparent cycle lengths for those intervals. If you miss a period entry, you can add it retroactively by typing the date in the "Period start date" field — entries are sorted by date automatically.