Race Time Predictor
Enter a recent race result and target distance to estimate your finish time using the proven Riegel formula.
How to use the race time predictor
- Enter the distance and finish time of a recent race.
- Choose the target distance you want to predict.
- Click Predict to estimate your finish time.
Why pace slows over longer distances
The Riegel exponent of 1.06 captures the physiological reality that you cannot hold 5K pace for a marathon. Endurance, fueling, and fatigue resistance all matter more as distance increases, which is why longer predictions assume adequate training.
Why the Riegel exponent matters
Riegel's formula uses an exponent of 1.06 to model how pace fades as distance grows. That single number encodes a hard truth: you cannot hold 5K pace for a marathon. The further you extrapolate from your input race, the more the prediction depends on endurance you may not yet have.
A 5K predicting a 10K is trustworthy. A 5K predicting a marathon is optimistic by design, because the formula assumes you have done the long runs that make the longer distance possible. Treat short-to-long predictions as a ceiling, not a plan.
Making predictions more accurate
Feed the predictor your most recent, best-effort race over a distance close to your target. A parkrun 5K from last weekend predicts a 10K far better than a half marathon from two years ago. If you have several recent races, run the prediction from each and look for agreement; a tight cluster is a confident estimate, while a wide spread usually means your endurance and speed are unevenly developed.
Frequently asked questions
How accurate is a race time predictor?
Predictions are most accurate when the goal distance is close to your recent race and you have the endurance base to match. Predicting a marathon from a 5K tends to be optimistic without long-run training.
What formula does this predictor use?
It uses Pete Riegel's formula: T2 = T1 × (D2 ÷ D1)^1.06, which models how pace slows as distance grows.
How accurate is a marathon time predicted from a half marathon?
Reasonably accurate if you have done marathon-specific long runs, typically within a few minutes. Without that endurance base, the real marathon is usually slower than the prediction because late-race fatigue and fuelling become limiting factors the formula cannot see.
Can a race predictor account for hills or heat?
No. Predictions assume similar, neutral conditions. Add time for a hillier or hotter target race than the one you entered, and expect to be faster on a flatter, cooler course.
Which race distance should I enter for the best prediction?
Use a recent race as close as possible to your target distance. The smaller the gap between your input and target, the more reliable the result.
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