Race Time Predictor
Predict your race time at a new distance from a recent race result, using Riegel's formula.
- Free to use
- Accurate results
- No registration required
- Works on all devices
Your result
0:52:07
Predicted time
AI explanation
Formula
T2 = T1 × (D2/D1)^1.06 (Riegel's formula)Worked example
5K in 25:00 predicting a 10K
| Field | Value |
|---|---|
| Known race distance | 5 |
| Known race time | 25 |
| Target race distance | 10 |
| Predicted time | 0:52:07 |
Assumptions
- Uses Peter Riegel's 1977 formula, a widely used race-time prediction model.
- Accuracy degrades the more different the two distances are — predicting a marathon from a 5K time is much less reliable than predicting a 10K from a 5K time.
- Assumes similar training and conditions between the known and target races.
Frequently asked questions
How accurate is Riegel's formula?
It's reasonably accurate for predicting between similar distances (e.g. 5K to 10K), but becomes less reliable across very different distances (e.g. 5K to marathon), where endurance and fueling strategy play a much larger role than raw speed.
What is the 1.06 exponent in Riegel's formula?
It's an empirically derived "fatigue factor" reflecting how running pace naturally slows as distance increases — a value of exactly 1.0 would imply pace stays constant regardless of distance, which isn't realistic.
Can I use a training run instead of a race result?
Yes, but race-day adrenaline and pacing strategy typically make actual race times faster than equivalent training efforts, so a training-run prediction may be conservative.
Related calculators
Related articles
Sources
- Athletic Records and Human Endurance — American Scientist (Peter Riegel). Effective 01-05-1981, reviewed 13-09-2026.
This calculator provides a general estimate only.
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