Health & Fitness
Race Time Predictor - Predict Finish Times Using Riegel's Formula
Predict your finish time for any race distance based on a known race result. Uses Riegel's formula for 5K, 10K, half marathon, marathon, and more.
Predicted finish time
3:59:47
Show calculation steps
- Known race: 5 km in 25:00
- Target distance: 42.195 km
- Riegel formula: T₂ = 25:00 × (42.195 / 5)^1.06
- Predicted time: 3:59:47
- Pace: 5:41 /km · 9:09 /mile
Uses Riegel's formula: T₂ = T₁ × (D₂/D₁)^1.06. Predictions assume similar conditions and fitness level.
How race time prediction works
This calculator uses Riegel's formula, which predicts your finish time at a new distance based on a known race result. The formula is:
T₂ = T₁ × (D₂ / D₁)1.06
Where T₁ is your known time, D₁ is the known distance, D₂ is the target distance, and T₂ is the predicted time.
The fatigue factor (1.06)
The exponent 1.06 accounts for the fact that longer races take proportionally more time due to fatigue. A value of 1.0 would imply constant pace regardless of distance: the 1.06 exponent gives a more realistic prediction.
Accuracy
Riegel's formula works best for races between 3.5 km and 100 km. Predictions become less accurate for very short sprints or ultra-long distances. The formula also assumes similar training and conditions for both races.
Common use cases
- Predict your marathon time from a half marathon PR
- Set a realistic goal pace for your first 10K based on 5K results
- Compare performance across different race distances
Cameron’s formula
An alternative to Riegel’s formula is Cameron’s model, which uses empirically derived constants to account for physiological differences at very short and very long distances:
T₂ = T₁ × (D₂ / D₁) / (a + b × D₁^c) / (a + b × D₂^c)Cameron’s formula tends to be more accurate than Riegel’s for predictions that span very different distances (e.g., 5K to marathon), at the cost of added complexity. Use Riegel for adjacent distances; consider Cameron for large jumps.
Training volume consideration
Riegel’s formula assumes equivalent training for both the reference race and the predicted race. If you have significantly increased weekly mileage, added structured tempo runs, or improved your VO2 max since your reference result, your actual performance may outperform the prediction. Conversely, predicting a marathon from a 5K trained peak without building long-run base will likely underperform.
Race equivalence table
Predicted times for a runner with a 25:00 5K (Riegel’s formula):
| Distance | Predicted time | Target pace (per mile) |
|---|---|---|
| 5K (3.1 mi) | 25:00 | 8:03/mi |
| 10K (6.2 mi) | 52:03 | 8:23/mi |
| Half marathon (13.1 mi) | 1:55:41 | 8:50/mi |
| Marathon (26.2 mi) | 4:01:30 | 9:14/mi |