Riegel Race Time Calculator
Predict your race time at any distance from a recent result using Peter Riegel’s formula T₂ = T₁ × (D₂ ÷ D₁)^1.06 — with the equivalent pace.
How it works
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Peter Riegel’s 1977 model captures a simple truth of endurance racing: as distance grows, sustainable pace fades in a predictable way. The exponent 1.06 means doubling the distance costs about 6% in pace, so predictions stay realistic across the 5K-to-marathon range. It is most accurate when the two distances are reasonably close and you are equally trained for both — projecting a marathon from a 5K assumes endurance you may not yet have.
Worked example
A 45:00 10K is 2,700 seconds. Half marathon (21.0975 km): 2,700 × (21.0975 ÷ 10)^1.06 = 2,700 × 2.209 ≈ 5,964 s — about 1:39:24, a 4:43/km pace versus 4:30/km for the 10K.
Sources: Riegel, Peter (endurance model)
Frequently asked questions
Why 1.06 and not just scaling time linearly?
A linear scale would assume you hold the same pace forever, which no one does. The 1.06 exponent adds the real-world fade: you cannot run a marathon at your 5K pace, and Riegel quantifies exactly how much you slow.
How far apart can the distances be?
It works from roughly 1.5 km to the marathon, but accuracy is best when the target is within about 3–4× the known distance and you have trained the endurance. Predicting a marathon from a mile is mathematically fine but athletically optimistic.
Can I predict a shorter race from a longer one?
Yes — the formula runs both ways. Predicting a 5K from a half marathon tends to be conservative, because most runners have more raw speed than their long-distance result implies.
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This calculator is for educational purposes. Double-check important results before acting on them.