Race Time Predictor
Enter a recent race distance and finish time, plus a target distance, to predict your likely finish time at that new distance, using the widely used Riegel formula.
How the Race Time Predictor formula works
The Riegel formula, commonly used to predict race times across distances:
Predicted time = Known time × (Target distance / Known distance)^1.06
Step-by-step calculation
- Divide the target distance by your known race distance.
- Raise that ratio to the power of 1.06, a commonly used fatigue exponent.
- Multiply by your known finish time to get the predicted time at the new distance.
Worked example
A 10K finished in 50 minutes, predicting a half marathon (13.1 miles ≈ 21.1 km) time: Predicted = 50 × (21.1/10)^1.06 ≈ 50 × 2.24 ≈ 112 minutes, or about 1 hour 52 minutes.
Frequently asked questions
How accurate is the Riegel formula?
It's a widely used, reasonably reliable estimate for predicting times between moderately different distances (like 5K to 10K, or 10K to half marathon), especially for well-trained runners — predictions across very different distances (like a 5K time predicting a full marathon) tend to be less reliable, since endurance and pacing strategy differ more at longer distances.
Does this account for training between now and the target race?
No — this predicts based purely on current fitness as reflected in the known result. Additional training, or lack of it, before the target race will shift actual performance relative to this prediction.