Track all day activity. SmartTrack automatically recognizes workouts. Optical Heart Rate monitor. Cardio Fitness Level. Guided breathing sessions. Water resistant to 50 M for pool or rain wear. Receive smartphone notifications. Connect to smartphone GPS for real-time. Battery lasts 4+ days. 15+ Exercise modes. Female Health Tracking. Fitbit app. Wirelessly syncs to iOS Android and Windows devices. Small band fits wrists 5.5"-7.1" circumference. Large band fits wrists 7.1"-8.7" circumference.