Running race time predictor
Estimate your time at another distance from a race you've already run (Riegel formula).
Riegel formula: it assumes similar training and a small fatigue penalty over longer distances. A realistic target, not a guarantee.
How the calculation works
The Riegel formula predicts your time at a new distance: T₂ = T₁ × (D₂ ÷ D₁)1.06. The exponent 1.06 (above 1) reflects that pace drops slightly over longer distances. E.g., 5 km in 25 min predicts ~52 min for 10 km.
Examples
- 5 km in 25 min predicts ~52 min for 10 km.
- 10 km in 50 min predicts ~1h50 for a half marathon (21.1 km).
Frequently asked questions
Does it work for the marathon?
It does, but the bigger the distance jump, the less exact — the marathon depends heavily on endurance and race strategy.
Why is the exponent 1.06 and not 1?
If it were 1, pace would be constant. The 1.06 reflects that pace drops slightly over longer distances due to fatigue.
Does the prediction replace training?
No. It assumes you are trained for the new distance. Without specific preparation, the real time is worse.
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Updated on June 30, 2026 · by Rafael Rossi · Methodology & sources