Riegel's Race Prediction Formula
Pete Riegel was a research engineer in Ohio who published a groundbreaking study in Runner's World magazine in 1977. His formula, T2 = T1 × (D2/D1)^1.06, discovered a consistent mathematical pattern between race distances and finish times. Nearly 50 years later, it remains the most widely used race prediction method worldwide — including in Aerix's Race Predictor.
The Formula
T2 = T1 × (D2 / D1)^1.06
T2
Predicted finish time
T1
Known race time
D2
Target race distance
D1
Known race distance
The Fatigue Factor: Why 1.06?
If running were purely aerobic with zero fatigue, a 25:00 5K runner would finish 10K in exactly 50:00 — a linear doubling (exponent 1.0). But real running involves glycogen depletion, neuromuscular fatigue, thermoregulation stress, and pacing errors. Riegel analyzed race data from sprints to ultramarathons and found that 1.06 best captures this cumulative fatigue across all distances. Runners who fade badly in long races may fit 1.07–1.08 better. Ultra specialists often beat the prediction. For most runners racing 5K to marathon, 1.06 is the most reliable value.
Worked Examples
5K → 10K
Recent 5K: 25:00
≈ 52:07
10K → Half Marathon
Recent 10K: 48:00
≈ 1:45:54
Half → Marathon
Recent Half: 1:55:00
≈ 3:59:46
Prediction Table — From 5K
| 5K | 10K | Half Marathon | Marathon |
|---|---|---|---|
| 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 22:30 | 46:55 | 1:43:30 | 3:35:48 |
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Prediction Table — From 10K
| 10K | Half Marathon | Marathon |
|---|---|---|
| 40:00 | 1:28:20 | 3:04:05 |
| 45:00 | 1:39:23 | 3:27:05 |
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Limitations
Large distance gaps: predicting marathon from a 1-mile time has high error. Best when distance ratio is 3:1 or less.
Untrained runners: beginners without aerobic base for longer distances will underperform predictions significantly.
Ultramarathons: beyond marathon distance, sleep deprivation, elevation, and terrain make the formula unreliable.
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Alternative Prediction Methods
Cameron Formula
Adjusts the exponent for specific distance pairs instead of using a fixed 1.06. Can be more accurate for large distance jumps.
Purdy Points
A point-based system that standardizes race performances across distances and genders. Complex but useful for comparing results.
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FAQ
Q. Is doubling the half marathon time a good marathon estimate?
It's optimistic. Riegel's formula predicts ~10 minutes slower than a simple doubling for a 2-hour half marathoner. The added distance brings exponential fatigue that a linear estimate ignores.
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