This app calculates your average heart rate / running speed and interpolates it to the maximal speed in which you would complete a 10KM race. This calculation is performed by calculating the running speed/ HR versus your aerobic treshold heart rate (mostly 91% of the maximal heart rate), deriving your aerobic treshold speed (on a 15KM run), to finally correct it to the 10KM running speed using Riegel's formula. Riegel's formula states that your speed decreases with 6% everytime the distance is doubled, or in time: T_Projected = T_Actual * ((Distance_actual / Distance_Projected) ^1,06) (A 10KM race is run at 103% of your aerobic treshold heart rate, while a 15kM race is run at 100%) Please note that this is an approximation and it assumes sufficient (endurance) training