TeenCycle is a private period tracking application designed specifically for teenagers. The app operates entirely offline, storing all user data locally on the device with no cloud synchronization, no user accounts, and no tracking pixels. Its primary function is to allow users to log menstrual cycle days and receive predictions for future periods using a minimal interface. The app was developed by a family for their own daughter and does not require an email address or any personal information to use. Main features include a single tap logging system where users press a circle to record their period day, two flow state options, and a three screen interface that shows the current cycle day, prediction for the next period, and a simple calendar. The app contains no social feeds, habit streaks, push notifications, or advertisements. It does not ask any personal questions beyond the cycle data itself. Technical requirements are minimal: the app runs on a mobile device and functions without an internet connection. A 7 day free trial is available, after which a one time payment of $9.99 unlocks the full app with no subscription or recurring charges. Typical use cases involve a teenager opening the app only when needed to log a period day, then closing it without further interaction. The app predicts the next period based on logged history. Because no account creation or data sharing is involved, the user’s information remains private and inaccessible to the developers. The design intentionally excludes features that could distract or collect personal data, focusing solely on tracking and prediction. The app is built with privacy by design principles and does not connect to any servers.