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Race time predictor
5K
21:35–21:42
Riegel 21:35 · VDOT 21:42
10K
45:00
Half marathon
1:39:17–1:39:44
Riegel 1:39:17 · VDOT 1:39:44
Marathon
3:27:00–3:27:15
Riegel 3:27:00 · VDOT 3:27:15
How to use it
Enter the distance and time of a recent all-out race. Below is a prediction for standard distances from two different models — where they disagree, that's a real signal the prediction is less reliable.
Formula and source
Riegel: T₂ = T₁ × (D₂/D₁)^1.06 (1.08 beyond marathon)
VDOT: per Daniels & Gilbert (1979), numerical solve
Pete Riegel, 1977 · Daniels & Gilbert, 1979 (the same equations behind Jack Daniels' VDOT tables)
Worked example
10K in 45:00 → Riegel predicts a marathon of ≈ 3:29:xx, VDOT ≈ 3:26:xx. A gap of a few minutes is normal, not a calculation error.
Limitations
- Accuracy is validated for 3.5–230 minute efforts (roughly 1500m–marathon); outside that range, trust the prediction less.
- The source result should be recent (4–6 weeks) and all-out — an easy run will underestimate the prediction.
- The prediction assumes adequate training for the target distance (predicting a marathon from a 5K overrates long-distance readiness).
- For ultras (beyond marathon) we already raise the Riegel exponent to 1.08 — still treat the number as a ballpark, not a guarantee.