When your heart beats, your capillaries expand and contract based on blood volume changes. PurePulse LEDs (light-emitting diodes) on your Fitbit device reflect onto the skin to detect blood volume changes, and finely-tuned algorithms are applied to measure heart rate automatically and continuously. The heart-rate icon you see on the display tells you if you're in 1 of 3 heart-rate zones.
Fitbit devices calculate distance by multiplying your walking steps by your walking stride length and multiplying your running steps by your running stride length. It estimate your stride lengths using your height and sex.
HOW FITBITS CALCULATE HOW MANY CALORIES YOU BURN A DAY
Fitbit devices estimate your calorie burn based on your age, sex, height, and weight. If your device measures heart rate, the calorie burn estimate also takes heart rate into account. Note that the value you see on your Fitbit device when you wake up in the morning is your estimated calorie burn for the day so far. You still burn calories even if you are sedentary or sleeping.
Fitbit’s are set to track your sleep automatically.Fitbits algorithms assume sleep has begun when you haven’t moved in more than an hour. In ‘normal’ mode, Fitbit will only track major movements such as rolling over. In ‘sensitive’ mode, Fitbit will log almost all movements as ‘restless’ time.