Riegel's formula: T2 = T1 × (D2/D1)^1.06. Predict any race time from a known performance.
A 25-minute 5K predicts a 2:03 marathon. A 45-minute 10K predicts a 1:35 half marathon.
The formula assumes consistent training and fitness level across both distances.
Riegel's formula is accurate within 5–10 minutes for most trained runners. Accuracy decreases for untrained or very slow runners.
T2 = T1 × (D2/D1)^1.06, where T1 is your known race time, D1 the distance, and D2 the target distance.