Race Time Predictor

The Riegel Formula (1977)

Peter Riegel published this endurance prediction model in 1977. T1 is your known race time, D1 is that race's distance, and D2 is the distance you want to predict. The 1.06 exponent is the fatigue factor — it captures how performance degrades as distance increases.

Why 1.06? The Fatigue Factor

If you double the distance, your time more than doubles. The 1.06 exponent accounts for the cumulative cost of endurance — glycogen depletion, muscle fatigue, heat build-up, and mental strain all compound over longer efforts. A 20-minute 5K runner won't simply run a 40-minute 10K; the formula predicts roughly 41:30 instead.

Limitations

• Training specificity: Predictions assume you have trained appropriately for the target distance. A fast 5K runner who has never run more than 10 km will likely be slower than the predicted marathon time.
• Distance range: The formula is most accurate when the known and target distances are within 2-3x of each other. Predicting a marathon from a 1-mile time is less reliable than predicting it from a half marathon.
• External factors: Terrain, weather, altitude, and nutrition strategy all affect longer races disproportionately. A flat 10K on a cool morning tells you less about a hilly, humid marathon.
• Race-day execution: Pacing, fuelling, and hydration become increasingly critical as distances grow. The formula cannot account for hitting the wall at kilometre 32.