Fitbit inspire HR is a friendly heart rate fitness tracker for every day that helps you build healthy habits. encouraging companion motivates you to reach your weight and fitness goals and even enjoy the journey with 24/7 heart rate, workout features, calorie burn tracking, goal celebrations, sleep stages and up to 5 days of battery life. Large fits wrist 7.1" to " 8.7"" in circumference. Small fits wrist 5.5" to " 7.1"". Syncs with iPhone 4s and later, iPad 3 gen and later, Android 5.0 and later and windows 10 device. Features & details Utilize 24x7 heart rate to accurately track calorie burn, resting heart rate & heart rate zones during workouts Track all day activity, including steps, distance, hourly activity, active minutes and calories burned. The Fitbit inspire band is made of a flexible, durable elastomer material similar to that used in many sport watches, and fastens with a peg and loop Automatically track sleep plus due to the heart rate feature and get more insights into your light & deep sleep stages Enjoy up to 5 days of battery for daily progress without constant charging. Automatically and conveniently record workouts like walks, swimming & bike rides in a snap with smart track. Radio transceiver: Bluetooth 4.0. Fitbit inspire and inspire HR are water resistant to 50 meters. Do not recommend wearing inspire or inspire HR in a hot tub/sauna. Operating temperature: 14 degree to 113 degree Fahrenheit. Max operating altitude: 28,000 feet