Features include: PurePulse Heart Rate; Smartphone Notifications; All-Day Activity; Auto Sleep Tracking and Silent Alarms; Interchangeable Accessories; Reminders to Move; Sleep Stages; Long Battery Life; Tap display; Syn's Wirelessly

Battery Up to 7 Days: A single charge gives you up to 7 days of battery life, so you can plug in less, and move more.

SmartTrack Auto Exercise Recognition. It auto-detects workouts like running, elliptical and biking and logs them to your dashboard

Auto Sleep Tracking & Alarms. Automatically track your sleep duration and consistency. Then wake up peacefully with a silent alarm.

Steps, Calories & Distance. Track your all-day activity, and tap the OLED display to see how close you are to your daily goal.Call, Text & Calendar Alerts. Receive call, text and calendar notifications on display, so you never miss the messages that matter.

Customizable Clock Faces. Choose your clock display based on the stats you want to see and the design that suits your taste.Reminders to Move. Get friendly on-screen messages that encourage you to reach mini-goals of 250 steps each hour.

Small size fits wrists 14–17cm in circumference
Large size (sold separately) fits wrists 17–20.6 cm in circumference

Which phones and tablets can I use with my Fitbit watch or tracker:
https://help.fitbit.com/…article/2315