Track steps, distance, calories burned and active minutes See stats and time with a bright OLED tap display;Battery life up to 5 days Battery life and charge cycles vary with use, settings, and other factors; actual results will vary Radio transceiver: Bluetooth 40 Automatically track how long and how well you sleep, and set a silent, vibrating alarm ; The Fitbit Alta wristband is made of a flexible, durable elastomer material similar to that used in many sports watches The Alta tracker and clasp are made with surgical-grade stainless steel ; Syncs Wirelessly : Android and Windows devices Personalize with interchangeable metal, leather and classic bands sold separately ; Battery life up to 5 days Battery life and charge cycles vary with use, settings, and other factors; actual results will vary Battery type: Lithium-polymer Charge time: One to two hours Get call, text and calendar notifications at a glance when phone is nearby Buy with confidence when your Fitbit product 'ships from and is sold by' the following authorized Fitbit retailers:  Amazon.com, Heart Rate Monitors USA and Clever Training. Fitbit does not guarantee the quality or authenticity of products purchased from non-authorized resellers on Amazon, and they are not eligible for warranty and replacement from Fitbit. The minimum and maximum circumference range between 170 - 206 millimeters