Free Tool
Race Time Predictor
Have one recent race in the bag? Enter its distance and time, pick a target distance, and get a predicted finish — calculated with Peter Riegel's well-known endurance formula.
Predicted finish time
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Predicted pace
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min/km · — /mile
Enter a recent race and a target distance to see the prediction. It updates as you type.
How the prediction is calculated
This uses Peter Riegel's endurance formula, published in American Scientist in 1981:
T₂ = T₁ × (D₂ ÷ D₁)1.06
T₁ and D₁ are the time and distance of the race you ran; T₂ is the predicted time for the new distance D₂. The exponent 1.06 is Riegel's fatigue factor — it captures that pace slows as distance grows. If pace held perfectly, the exponent would be exactly 1; the extra 0.06 is the empirically observed slowdown for endurance events.
Accuracy caveats. Riegel's formula works best when the two distances aren't too far apart (roughly a 3–4× range) and assumes matched training and comparable conditions. It tends to be optimistic for a first marathon predicted from a short race, since it can't know your endurance or fueling. Treat the result as a well-grounded estimate, not a guarantee. Source: Riegel, P.S. (1981), "Athletic Records and Human Endurance," American Scientist 69(3).