Race Time Predictor

About this calculator

The Riegel formula, developed by exercise physiologist Pete Riegel in 1977, predicts a runner's finish time at one distance from a known result at another: T2 = T1 × (D2/D1)^1.06, where the exponent 1.06 captures the fact that pace naturally slows as distance increases — running twice as far takes more than twice as long, even at consistent fitness, because fatigue and glycogen depletion compound over longer efforts. It's remarkably accurate for predicting between reasonably similar distances (a 10K time predicting a half marathon, for example) but grows less reliable across very different distance ranges, since factors like fueling, pacing strategy and heat management matter increasingly at marathon-plus distances.

Runners and coaches use Riegel-based predictions to set realistic goal times for an upcoming race based on a recent tune-up race or time trial, to gauge fitness progress between training blocks, and to choose appropriate pacing splits going into race day. It's one of several prediction models (alongside the VDOT and Cameron formulas) that coaches cross-check against each other, since no single formula perfectly captures every runner's individual fatigue profile.

This calculator takes a recent race result and a target distance and predicts your finish time using the Riegel formula.