Question 1

How accurate are race time predictors?

Accepted Answer

The Riegel formula is accurate within 3–5% when predicting similar distances — for example, using a 10K time to predict a half marathon. Accuracy drops for large distance jumps (e.g., 5K to marathon) because factors like fueling, muscle fatigue, and pacing strategy compound over longer distances. Use a recent race effort for best results.

Question 2

What is the Riegel race prediction formula?

Accepted Answer

The Riegel formula is T2 = T1 × (D2/D1)^1.06. T1 is your known race time, D1 is the known distance, D2 is the target distance, and the exponent 1.06 is the fatigue factor — reflecting that performance degrades as distance increases. For example, a 25:00 5K predicts approximately 52:00 for a 10K using this formula.

Question 3

How do you predict marathon time from a half marathon?

Accepted Answer

Enter your half marathon finish time as the known time, set Known Distance to Half Marathon, and set Target Distance to Marathon. The Riegel formula will multiply your time by approximately 2.11, accounting for the additional fatigue of the second half. A 1:45 half marathon predicts roughly a 3:41 marathon. Use the Average method to blend Riegel and Cameron predictions for a conservative estimate.

Race Finish Time Predictor

Your Known Race

Predicted Times — All Distances

Race Finish Time Predictor

Your Known Race

Predicted Times — All Distances

Related Calculators