Race Time Predictor (Riegel Formula)
By Santaji GadepatilinUpdated October 27, 2026
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How to use this calculator
Enter a recent race distance and time and your target distance. The predicted finish time is calculated in minutes (switch to hours if you prefer).
Formula
T₂ = T₁ × (D₂ ÷ D₁)1.06
Example
A 25:00 5K predicts:
| Race | Predicted time |
|---|---|
| 10K | 52.1 min |
| Half marathon | 115.0 min (1.92 h) |
| Marathon | 239.8 min (4.00 h) |
Frequently asked questions
What is the Riegel formula?
T₂ = T₁ × (D₂ ÷ D₁)^1.06, published by Peter Riegel in 1977. It assumes you slow slightly as distance increases.
How accurate is it?
Reasonably good for well-trained runners between about 3 and 50 km. Marathon predictions are often optimistic for runners with low weekly mileage.
Which race should I use?
A recent, all-out effort over a distance close to your target gives the best prediction.