Predict an equivalent race time
This running race predictor gives a quick, transparent estimate from one completed result. Choose your known distance and finish time, then select a target race. It works as a race time calculator for familiar races and bounded custom distances without a profile, account or opaque score.
How the Riegel race time formula works
The calculation is T₂ = T₁ × (D₂ ÷ D₁)^1.06. It uses your entered elapsed seconds and canonical distance in metres, retaining the raw result until display rounding. The fixed exponent is 1.06, so a 5k to 10k race predictor and a 10k to half marathon predictor apply exactly the same reproducible method.
How reliable is a race time prediction?
Riegel is a population-style extrapolation, not an individual physiological model. Fatigue resistance, training, course, weather, pacing and preparation may make your actual time faster or slower. A 5k to half marathon predictor or 10k to marathon predictor has a larger gap than nearby races, so treat it as a broad estimate. The same applies to a half marathon to marathon predictor.
Worked example
A 10K in 50:00 projects to a half marathon in 1:50:19. The raw formula result is rounded once for display; the pace is calculated from the unrounded result. It is an estimate rather than a target guarantee.
FAQ
How does this race time predictor work?
It applies the fixed Riegel formula to one completed race time and distance, then estimates an equivalent time for your selected target distance.
Is a predicted race time guaranteed?
No. It is an estimate, not a guaranteed race result. Training, fatigue resistance, course, weather and race preparation can materially change your outcome.
Can I predict a 5K from a marathon?
Yes. The formula supports both interpolation and extrapolation, including predicting a shorter race from a marathon.
Why is 5K to marathon less reliable?
It is a large distance gap, so endurance, training and pacing differences matter more than they do between nearby distances.
What Riegel exponent does EnduraCalc use?
EnduraCalc uses the fixed Riegel exponent 1.06 for every prediction.
Can I enter a custom race distance?
Yes. Custom distances from 400 metres to 100,000 metres are supported in kilometres or miles.