How Race Time Prediction Works
This tool uses the Riegel formula, one of the most widely accepted methods for predicting race performance across distances. Developed by researcher Pete Riegel in 1977 and published in Runner's World, the formula models the relationship between race distance and finishing time as a power law.
The formula is T2 = T1 × (D2 / D1)1.06, where T1 is your known finishing time, D1 is the known distance, D2 is the target distance, and 1.06 is the fatigue factor that accounts for the increasing physiological cost of running longer distances. In practice, this means doubling the distance adds roughly 10% more time than a simple linear estimate.
For best accuracy, use a recent race result from a distance as close as possible to your target. Predictions between neighbouring distances (e.g., 10K to Half Marathon) tend to be more reliable than extreme jumps (e.g., 5K to Marathon), where factors like fueling strategy and endurance training play a larger role.