Use this running calculator if you want to know how well you could do in one race, based on the results of another race.
A guy called Pete Riegel developed this calculator in the seventies and refined it in the eighties.
Results of this calculator are often surprisingly accurate.
However....
This calculator tells you how fast you can run in another race, given you put in the needed training.
So don't expect to be able to run a marathon within three hours, just because you can run a 10 K in 38 minutes. The calculator merely tells you it is within your ability to run a marathon this fast.
Especially when using this calculator for (much) longer distances there is a risk involved :
In shorter races other body systems are used. The longer the race, the more your aerobic capacity is used. So don't compare a 100 metre race (fully anaerobic) with a 10K.
In order to be able to get to the calculated times in longer races your base needs to be well developed. It takes years to fully develop your base.
So, use this calculator wisely ! Use it as a gauge of your abilities not necessarily as the goal time for your next race.
By the way, the formula used by Riegel is the following:
T2 = T1 * (D2 / D1) 1.06
Where:
D1 = Distance previous race
D2 = Distance next race
T1 = Time run in previous race
T2 = Predicted time for next race