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Running Race Time Predictor
Predict your 5K, 10K, half, or marathon from a recent race.
The formula
This uses Pete Riegel’s endurance model, the same one behind most race calculators: T₂ = T₁ × (D₂ / D₁)1.06. The 1.06 exponent is the important part — it bakes in the reality that you cannot hold 5K pace for a marathon, so longer predictions come out proportionally slower.
Reading the result
Treat the prediction as a best-case for a day when you are rested, the course is fair, and you pace evenly. Predicting up from a short race (5K → marathon) tends to flatter you, because the marathon is limited by endurance and fueling the formula can’t measure. Predicting between nearby distances (10K → half) is usually close.
Questions
How does the prediction work?
It uses Pete Riegel’s endurance formula: T2 = T1 × (D2 ÷ D1)^1.06. The 1.06 exponent accounts for the fact that pace slows as distance grows, so a longer race is predicted to be slightly slower per mile than a short one.
How accurate is it?
It is most reliable when the two distances are within a few-fold of each other and you are equally well trained for both. Predicting a marathon from a 5K tends to be optimistic, because the marathon depends heavily on endurance and fueling the formula cannot see.
Should I race the predicted time?
Treat it as a ceiling for a perfect day, then set a goal slightly more conservative for your first attempt at a distance. Course profile, heat, and pacing discipline all move the real result.