Features & details Track steps, distance, calories burned and active minutes Ultra-thin, removable tracker hides in bands, pendants and bangles, The Fitbit Flex 2 wristband is made of a flexible, durable elastomer material similar to that used in many sports watches. It also has a surgical-grade stainless steel clasp Swim-proof for tracking swimming, life-proof for wear in the ocean, shower, pool and beyond SmartTrack automatically recognizes select workouts and records them to your Fitbit app; Get call and text notifications on your wrist with unique vibration patterns and color-coded LED lights Automatically track how long and how well you sleep, and set a silent alarm to wake with a small vibration; An LED display lights up when you get calls, texts and Reminders to Move, and shows you how close you are to your daily goal with a simple tap. Operating temperature: 14 degree to 113 degree fahrenheit, Maximum operating altitude: 30,000 feet The minimum and maximum circumference range between 140 - 206 millimeters