Race Time Predictor

How the Riegel Formula Works

The Race Time Predictor uses Pete Riegel's endurance formula to estimate your finish time at a new distance based on a recent race result. The formula is:

T2 = T1 × (D2 / D1)^1.06

Here T1 is your known finish time over distance D1, and T2 is the predicted time over the target distance D2 (both distances in the same unit). The exponent 1.06 is the "fatigue factor" that accounts for the fact that you naturally slow down as the distance increases. If running speed were perfectly constant, the exponent would be 1.0; the extra 0.06 captures the gradual loss of pace over longer events.

Accuracy and Limitations

The Riegel formula works best when the target distance is reasonably close to the known distance — for example predicting a 10K from a 5K, or a half marathon from a 10K. Predictions become less reliable as the gap between distances grows.

• It assumes consistent, race-appropriate training and effort.
• It does not account for course terrain, hills, heat, or wind.
• For the marathon, many runners "hit the wall," so the prediction may be optimistic.
• Beginners and runners with limited endurance often run longer races slower than predicted.

Common Race Distances

• 5K = 5 km (3.107 mi)
• 10K = 10 km (6.214 mi)
• Half Marathon = 21.0975 km (13.109 mi)
• Marathon = 42.195 km (26.219 mi)

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