Race Time Predictor
Enter a recent race result to predict your finish time at any distance, from 1 km to 50K.
This calculator uses the Riegel formula (T₂ = T₁ × (D₂/D₁)1.06), published by Pete Riegel in Runner's World in 1977. It assumes aerobic performance degrades slightly as distance increases. The 1.06 exponent captures that fatigue is not perfectly linear.
Predictions are most accurate within two distance brackets of your input race (e.g. a 5K predicts a 10K well, but gets less reliable at marathon distance). Factors like heat, altitude, course profile, and individual endurance ratio can shift results by several percent.
Riegel uses a single fixed exponent (1.06) for everyone. But fatigue resistance is individual — in practice the exponent runs from about 1.04 for endurance-dominant runners to over 1.10 for speed types. Because the error compounds with the distance ratio, a 2% mismatch can swing a marathon predicted from a 5K by 5–10 minutes. And with only one race as input, a single hot, hilly, or off day skews every prediction.
A 5K and a marathon are limited by different systems. The 5K is capped by VO₂ max and lactate threshold; the marathon is capped by fat oxidation, glycogen storage, fuelling, and heat — none of which a short race can measure. Two runners with identical 5K times routinely finish 30+ minutes apart over 42 km. The formula also assumes you’ve trained the distance; it can’t know if you haven’t.
Vertical fits your personal endurance curve from hundreds of your runs, not one race against a textbook exponent. It reads your long-run history and how your pace holds up under fatigue, so the prediction reflects the engine you’ve actually built — and it updates every time you train.
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