In order to calculate its individual stride length, there are several possibilities. The easiest way to measure out the length of a single step is using a tape measure.
However, it is important to take into notice that steps never have the same stride and different lengths depending on the walking track or running styles. For this reason, the step length is determined for an average value over a certain distance.
Now you are looking at a distance, from which you exactly know how long it is or/and measure it with a pedometer. Then you walk this distance with the pedometer. The average step length is now calculated as follows:
Walking distance divided by the number of steps, needed for this distance.