Race Time Predictor
Predict your race finish time based on a recent result.
Enter MM:SS or H:MM:SS
Enter a race result above to see your predictions.
About the race time predictor
Predicting a finish time for one distance from a real result at another relies on Pete Riegel's widely used endurance-running formula: T₂ = T₁ × (D₂ / D₁)^1.06. Riegel developed it in 1977 after analysing thousands of race results across distances from 3 miles to beyond marathon, and it remains one of the most-cited prediction models in distance running.
Why the 1.06 exponent?
The exponent models how running speed naturally drops off over longer distances due to fatigue. 1.06 is the value Riegel found to fit a wide range of real race results, from 5K through marathon, reasonably well.
Worked example
A 10K finish of 45:00 (2700 seconds) predicts a half marathon time via T₂ = 2700 × (21.0975/10)^1.06 ≈ 5957 seconds, or about 1:39:17. The same formula scaled down predicts a 5K of roughly 21:35 — always check the prediction against how the training leading into your target race has actually gone.
Limitations
Predictions are most accurate when the two distances are reasonably close and your training matches the target distance. For example, a 5K time-trial predicts a 10K well, but is a rougher estimate for a marathon if you have not done any long runs. Terrain, weather, and pacing strategy on race day are not accounted for.
Riegel vs other models
Riegel is not the only prediction method — VDOT-based tables (see the Training Paces calculator) and Cameron's formula are common alternatives. Riegel tends to be more optimistic than VDOT for marathon predictions from short-distance results, because it does not separately model the endurance demands of very long races. When the two disagree by more than a few minutes, lean towards the more conservative estimate.
When to re-test
A prediction is only as good as the input time. If your most recent race result is more than a few months old, or your training volume has changed significantly since then, a fresh time-trial at 5K or 10K will give a more reliable base for predicting your next target race.