Race Predictor
Riegel formula: T2 = T1 × (D2/D1)^1.06. Predict marathon, half, 10K from any race. Pete Riegel 1977. Free calculator.
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📊 Riegel Predictions
📈 Exponent Impact
🥧 Distances
🏁 Known vs Predicted Pace
For educational and informational purposes only. Verify with a qualified professional.
Riegel formula (1977): T2 = T1 × (D2/D1)^1.06. Predict race time at any distance from a known race. The 1.06 exponent is the fatigue factor—works best for 1500m to marathon. Accuracy typically 2–4%.
Sources: Runner's World, Riegel 1977.
Key Takeaways
- • T2 = T1 × (D2/D1)^1.06. Use a recent race within 2–3 months.
- • 1.06 = fatigue factor. Longer races slow disproportionately.
- • Best for 1500m–marathon. Add 2–5% for first-time longer races.
- • Compare with Magic Mile or VDOT for cross-validation.
Did You Know?
How Does the Riegel Formula Work?
The Formula
T2 = T1 × (D2/D1)^1.06. T1, D1 = known time and distance. T2, D2 = predicted time and distance. Exponent 1.06 from empirical data.
Fatigue Factor
1.06 means doubling distance increases time by more than 2×. Marathon pace is ~10–15% slower than 5K pace. The exponent captures this.
Best Practices
Use a recent race (within 2–3 months). Similar conditions. For first marathon, add 2–5% buffer. Cross-check with other predictors.
Expert Tips
Sample Predictions (Riegel)
| Known | Marathon |
|---|---|
| 25 min 5K | ~3:52 |
| 1:45 half | ~3:38 |
| 50 min 10K | ~3:42 |
Frequently Asked Questions
What is the Riegel formula?
T2 = T1 × (D2/D1)^1.06. Pete Riegel 1977. Predicts race time at distance D2 from known time T1 at distance D1. The 1.06 exponent is the fatigue factor—longer races slow disproportionately.
How accurate is the Riegel predictor?
Typically within 2–4% for distances 1500m to marathon when using a recent, comparable effort. Works best when the known race is similar in intensity. Short sprints and ultra distances are less accurate.
What does the 1.06 exponent mean?
The 1.06 is the fatigue factor. It reflects that doubling distance does not double time—you slow down. 1.06 fits empirical data for 1500m–marathon. Different exponents (1.07, 1.08) are sometimes used for ultras.
Can I predict marathon from 5K?
Yes. T_marathon = T_5K × (26.2/3.10686)^1.06. A 25 min 5K predicts ~3:52 marathon. Add 2–5% for first-time marathoners; experience improves efficiency.
What distances does Riegel work best for?
Best for 1500m to marathon. Below 1500m, anaerobic contribution increases. Above marathon, fatigue models differ. Use a recent race within 2–3 months for accuracy.
How does Riegel compare to Magic Mile?
Riegel uses any known race; Magic Mile uses a 1-mile time trial. Riegel is more flexible. Magic Mile is Galloway-specific for run-walk. Both are estimates; actual performance varies.
Key Statistics
Official Data Sources
⚠️ Disclaimer: Riegel predictions are estimates. Actual performance varies with training, conditions, and pacing. Not coaching advice.