Online Tool Store
Period Cycle Tracker
Predict your next period, fertile window, and upcoming cycles from your last period date and average cycle length, saved only in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Example date shown — saved only in this browser (local storage) once you click Save, and nothing is ever sent anywhere. This is an estimate based on averages, not a medical prediction.
How the period cycle tracker works
- Enter the first day of your last period, your average cycle length, and your average period length.
- Click Save and predict — the tool estimates your next period, fertile window, and the next three predicted cycles.
- Your entries are saved in this browser so you don't need to re-enter them next time you visit.
The method
next period = last period start + average cycle length
Every future prediction is built by repeating that same addition — assuming your cycle stays close to the average length you entered. Ovulation and the fertile window are estimated relative to the predicted next period, using the widely-cited "about 14 days before the next period" pattern.
FAQ
How accurate are these predictions?
They're estimates based on average cycle length, assuming a regular cycle — real cycles vary in length from month to month for many people, especially due to stress, illness, travel, or hormonal changes, so treat these as a helpful guide rather than a guarantee.
How is the fertile window calculated?
It's estimated around ovulation, which typically occurs about 14 days before the next period starts — the fertile window shown spans roughly 5 days before to 4 days after that estimated ovulation date, based on common sperm and egg viability windows.
Where is my data saved?
Only in this browser's local storage, on this device — there's no account, no sync, and no server involved. Clear it any time with the "Clear saved data" button, or it disappears if you clear your browser's site data.
Can I use this for birth control or fertility planning?
No — this is a simple estimate for general awareness, not a clinically validated fertility awareness method. Talk to a healthcare provider for reliable family planning guidance.
How we compare
| Feature | Online Tool Store | Period tracking apps | A paper calendar |
|---|---|---|---|
| No account or app install | ✓ | App install, often an account | ✓ |
| Automatic future predictions | ✓ | ✓ | Manual calculation |
| Data stays on your device, not a company's cloud | ✓ | Often cloud-synced | ✓ |
For quick predictions without creating an account or syncing personal health data to the cloud, the Online Tool Store Period Cycle Tracker keeps everything local to your device.