In 2022, after Roe v. Wade was overturned, millions of women deleted their period tracking apps overnight. The fear was real: companies like Flo had already been caught sharing health data with Facebook. Suddenly, the app that was supposed to help you understand your body became a potential liability.
Three years later, most people went back to the same apps because the alternatives were worse — spreadsheets, paper calendars, or nothing at all.
There's a better option: a period tracker that never sends your data anywhere, because it runs entirely in your browser.
How browser-based tracking works
The Period Tracker stores everything in your browser's localStorage. That's a small database built into every web browser, accessible only from your device. No server receives your data. No company stores it. No API transmits it.
When you log a period — start date and end date — that information is written to localStorage and stays there. Close the tab, come back tomorrow, and it's still there. Clear your browser data, and it's gone forever. You have complete control.
Logging your cycle
The process is simple:
- Open the Period Tracker
- Enter the start date and end date of your period
- Click "Log"
That's one cycle. Log a few more, and the tool starts doing useful math.
What the predictions tell you
After two or more logged cycles, the tracker calculates:
Average cycle length — the number of days between the start of one period and the start of the next. Most people fall between 21 and 35 days. The tracker averages your actual data, not a textbook number.
Average period length — how many days your period typically lasts. Used to predict when the next one will end.
Next predicted period — based on your average cycle length from the last logged period start date.
Fertile window — estimated from the predicted ovulation date (roughly 14 days before the next period). The fertile window spans about 5 days around ovulation.
These predictions improve with more data. Two cycles give you a rough estimate. Six cycles give you something genuinely useful.
The calendar overlay
Here's where it gets powerful. If you open the Unified Calendar after logging cycles, your period data appears automatically:
- Red days — logged period days
- Pink days — predicted next period
- Green days — estimated fertile window
This means you can see your cycle overlaid with your events, habits, and countdowns. Planning a beach trip? Check if it overlaps with a predicted period. Have an important presentation? See if it falls in your typical low-energy PMS window.
What this tool is NOT
This is not a medical device. It uses simple averages, not machine learning or hormonal analysis. If your cycles are highly irregular, the predictions will be less reliable.
It's a personal awareness tool. It helps you notice patterns in your own body without handing that information to a corporation.
Your data, your rules
No account to create. No email to verify. No terms of service granting a company rights to your health data. Open the Period Tracker, log your cycles, and keep the information exactly where it belongs — on your device, under your control.