Race Time Predictor
Predict a race time at a new distance from a known result (Riegel formula).
Result
Predicted time (min)
52.1
Target pace (min/km)
5.21
How it works
This tool predicts your finish time at a new distance from a recent race result, using Riegel's formula: T2 = T1 × (D2/D1)^1.06. The exponent 1.06 captures a universal truth of endurance running — pace inevitably slows as distance grows, in a remarkably predictable way. Run a 10K in 50 minutes and the formula predicts about 1h51 for a half-marathon and 3h51 for a marathon. Note the marathon is more than four times the 10K time, not four times exactly: those extra minutes are the fatigue cost the exponent encodes. The prediction assumes you are properly trained for the target distance. A fast 5K runner who has never run long will not hold the predicted marathon pace — the formula extrapolates fitness, not preparation. Use recent races at nearby distances for the most reliable estimates.
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Frequently asked questions
How reliable is the prediction?
Good for nearby distances; the further the jump (e.g. 5K to marathon), the more it depends on endurance training.
Why the 1.06 exponent?
It reflects the slight slowdown as distance grows, fitted from real race data.
Why is my real marathon slower than the prediction?
Usually insufficient long-distance training. Riegel assumes equivalent preparation at both distances; without long runs, glycogen depletion slows you beyond the formula's fatigue factor.
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