Race Time Predictor
Equivalent race performance
Enter one result and choose a target race
Result
Calculation summary
Enter values to see the result
Prediction preview
From known result to target race
How to use this calculator
- 1Enter a recent representative race distance and elapsed finish time.
- 2Choose a target race preset or enter a custom target distance.
- 3Keep the standard 1.06 fatigue factor or adjust it when you deliberately want a sensitivity comparison.
Formula
T₂ = T₁ × (D₂ ÷ D₁)^b
T is finish time, D is distance, and b is the fatigue factor. The standard Riegel value used here is 1.06.
Calculation steps
- Convert the known and target distances to meters and the known finish time to seconds.
- Raise the distance ratio to the selected fatigue factor and multiply by the known time.
- Convert the predicted time into kilometer and mile pace and generate equivalent common-race projections.
Worked example
A 25:00 5K with a 1.06 fatigue factor predicts approximately 52:07 for 10K, or about 5:13 per kilometer.
Assumptions
- The known result represents current fitness and was run under reasonably comparable course, weather, and timing conditions.
- The model uses one performance and cannot evaluate training volume, endurance preparation, elevation, heat, wind, surface, or pacing strategy.
- Prediction uncertainty generally grows when the target distance is far from the known distance.
- The fatigue factor is a model parameter, not a personal physiological measurement.
Sources
Frequently asked questions
What result should I use as the known race?
Use a recent hard effort or race with reliable distance and elapsed time. A target distance close to that known race usually requires less extrapolation.
What does the 1.06 fatigue factor mean?
It makes predicted time increase slightly faster than distance. A factor of 1.00 would assume unchanged speed at every distance, while a larger value produces more slowdown at longer distances.
Why might the marathon prediction be too fast?
A short race does not prove marathon-specific endurance, fueling, or pacing preparation. Treat a long-distance projection as a comparison, not a guaranteed goal.
Is this a training plan?
No. It is a mathematical performance comparison and does not prescribe workouts, recovery, or safe race intensity.