Fitbit Charge 2Fitbit Flex 2
Tech specs 
SizeWidth: 21mm
Small: 14 cm - 17 cm in circumference
Large: 16.5 cm - 20.6 cm in circumference
XL: 20.6 cm - 23.6 cm in circumference
Width: 11 mm
Small: 14 cm - 17 cm in circumference
Large: 16.5 cm - 20.6 cm in circumference
MaterialStainless steel casing, elastomer strapThe Fitbit Flex 2 wristband is made of a flexible, durable elastomer material similar to that used in many sports watches. Flex 2 also has a surgical-grade stainless steel clasp.
ChargingUSB. USB. Charge time: One to two hours.
Battery lifeup to 5 daysup to 5 days
Water-resistanceSweat, rain and splash proof.up to 50 metres (swim-proof) - 5 ATM
Weight36 grams36 grams
SensorOptical heart rate tracker, 3-axis accelerometer, altimeter, vibration motor3-axis accelerometer, vibration motor
DisplayOLEDLED display with 5 indicator lights
Tap twice to see progress against your Main Goal
Screen size3.8 cm
ColorsBlack, Plum, Blue, Teal, Lavender Rose Gold, Black/Gunmetal Black, Lavender, Magenta, Navy.
Activity tracking 
Calories burnedYesYes
Heart rate24/7
Stress & recovery
Pulse Ox
Respiration rate
UV exposure
Skin temperature
Galvanic skin response
Hydration level
Calorie intake
GPSConnected GPS
Running features
Swimming featuresSwimming can be auto-tracked with duration and laps of four strokes monitored: freestyle, breaststroke, backstroke and butterfly. The app shows the amount of time you've swam, distance, pace and impact on your day.
Other Guided breathing
Non-fitness functions 
Smartphone notificationsYesCall and text notifications
On-board music storage
Connected pay
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