Race Time Predictor

What is the Riegel formula?

This calculator relies on the standard Riegel Formula: T2 = T1 * (D2 / D1)^1.06, where T1 is your source time, D1 is your source distance, and D2 is the target distance. It applies a small fatigue adjustment as distance increases and assumes consistent aerobic training appropriate for the target distance.

How accurate is a race time prediction?

A race prediction is a planning estimate, not a promise. It is most useful when your reference race and target race reflect similar fitness, training, terrain, weather, and pacing. Predicting a marathon from a fast 5K can overestimate your result if your long-run endurance has not caught up yet.

Can you predict a marathon from a 5K?

Yes—the calculator can project from any of the listed distances. Treat a long-distance projection as a starting point, then compare it with the endurance work you have actually completed. For aerobic-base context, see our Zone 2 running guide.