Polar H7 Bluetooth Soft Adjustable Strap Heart Rate Monitor, Size XS/Small. Removable pairs with devices for an accurate heart rate & calorie burn. Gently Worn, Works Great! HEART RATE MONITOR: Provides live, accurate heart rate to compatible mobile training apps through Bluetooth Smart technology FITNESS TRACKER: Waterproof heart rate sensor compatible with all Apple iPhone, iPads, and Android Devices STRONG BATTERY LIFE: For optimal battery life, detach transmitter from chest strap after every use ADVANCED TECHNOLOGY: Gym Link connects Polar heart rate monitors and activity trackers with compatible gym equipment CALORIE COUNTER CAPABILITIES: For those who want to count the calories they burn and get basic heart rate-based features to keep their fitness training simple