Riegel Race Time Predictor Calculator

About this calculator

This calculator applies Riegel's endurance formula, T2 = T1 × (D2/D1)^1.06, developed by exercise scientist Pete Riegel in 1977 from analyzing thousands of race results across distances. The exponent 1.06 (rather than 1.0) captures the fact that pace naturally slows over longer distances due to fatigue and fueling limits, so times don't scale linearly with distance.

Runners use it to set a realistic goal pace for an upcoming race based on a recent result at a different distance — predicting a marathon time from a half-marathon PR, for instance — though it works best between distances that aren't too far apart, since the formula assumes similarly trained endurance at both.