How Fitness Trackers Track Distance
Some more expensive Fitness Trackers have inbuilt GPS capabilities that mean they can use satellites to work out their position much like a car navigation unit does. These units should have a pretty accurate idea of how far you have traveled.
However, the GPS does need a lot of battery power, and GPS is not so good if you are inside a building. So this functionality is normally reserved for when you are doing a specific activity like a run or cycle.
The rest of the time these devices simply do what the non GPS devices do which is to count the number of steps taken and multiply the number of steps by your "Stride Length".
Therefore if you have walked 100 Steps and your Stride Length is 1 metre, you will have traveled 100 metres. However if your Stride length is 0.75 metres you will have only traveled 75 metres.
An important point here is that Stride Length does not affect the number of steps that you take, only how far those steps have taken you!
Your device will probably have a default estimate of your Stride Length based on your height and sex but you may also be able to enter more accurate measurements manually.
To find out your Stride Length you simply need to walk (as normal as possible) over a known distance and count the steps you took. Then divide the distance by the number of steps you took, to get your stride length. So if you walked 100 metres and took 125 steps your Stride Length would be 100 divided by 125 which equals 0.8 metres. You should also do this for running if your tracker allows for a separate Running Stride Length as this will be different to your walking Stride Length.
Note though that to be accurate with our language there are two types of terminology used here, Stride Length and Step Length. One is a measurement between one heel hitting the floor and the next heel hitting the floor, the other is the measurement between the SAME heel hitting the floor. So if your distance is wildly inaccurate (i.e. nearly double what it should be) after setting your Stride Length, try halving it.