Without GPS, the distance is calculated by multiplying average stride-length (walking and running) and the number of strides.

To get a more accurate estimate of the distance, the user should calibrate their stride-lengths.

Calibrating Walking or Running Stride-Length 
- Walk (or Run) with a normal stride a known distance (say 100 feet) and count the number of steps.

Stride length = Distance in Feet*12/Number of steps

The app will be able to compute distance more accurately if the user activates GPS tracking. But, in that case, the user MUST carry their phone with them.