A race time predictor estimates how fast you can run a target distance based on a known recent performance at another distance. If you recently ran a 10K in 50 minutes, the predictor can estimate your likely marathon finish time — saving you from the costly mistake of starting a marathon at an unrealistic pace.

This calculator uses the Riegel formula, developed by Peter Riegel and published in American Scientist (1981). The formula is: T2 = T1 × (D2/D1)^1.06. The exponent 1.06 captures the well-established fact that performance degrades at longer distances — a runner who is 10% faster over 5K than another runner will not be 10% faster over a marathon. The fatigue effect compounds with distance.

The Riegel formula is the most widely used prediction model in running, used by coaches, race organisers, and running apps worldwide. Its primary limitation is that it assumes similar training preparation for both distances. A 5K specialist who has not done long runs will underperform the prediction at marathon distance.