Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
The Riegel formula, developed by Pete Riegel in 1977, predicts a runner’s time at one distance from a known time at another: T2 = T1 × (D2/D1)^1.06. The 1.06 exponent accounts for the non-linear energy cost of running longer distances — each doubling of distance takes roughly 8% more time per kilometre than the previous. The formula is most accurate for trained runners who race regularly and least accurate for marathon prediction (where training specificity and glycogen availability introduce large individual variation).
- T2 = T1 × (D2/D1)^1.06
- 1.06 exponent accounts for endurance fatigue
- Accurate within 3-8% for trained runners
- Less accurate for marathon (glycogen wall)
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Riegel's formula estimates a runner's time at one distance from a known time at another, using a 'fatigue factor' exponent (commonly around 1.06) that accounts for how performance degrades over longer distances — while a useful quick estimate, the formula assumes a fixed fatigue factor that doesn't account for individual differences in endurance-specific training, so actual results can deviate meaningfully from the prediction.
FAQ
How does Riegel's formula work?
It takes a known race time and distance, then applies a fatigue exponent (typically around 1.06) to estimate the equivalent time at a different target distance.
Is Riegel's formula equally accurate for all runners?
No — it uses a generalized fatigue exponent that doesn't account for individual variation in endurance-specific training, so well-trained endurance specialists often outperform the prediction at longer distances, while speed-focused runners may underperform it.