What does this tool do?
Riegel’s model scales a known race performance by the ratio of distances raised to an exponent near 1.06. It is useful for a ballpark target when moving between 5K, 10K, half marathon and marathon.
The Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06Frequently Asked Questions
What is Riegel’s formula?
It predicts T₂ = T₁ × (D₂/D₁)^1.06, where T is time and D is distance. The 1.06 exponent reflects the way fatigue grows over longer races.
How reliable is a marathon prediction from a 5K?
It is rough. Endurance, fuelling and long-run experience matter far more as distance increases, so treat the result as a target rather than a guarantee.