Race Prediction Calculator
Predict your 10K, half, or marathon time from a recent race using the Riegel formula — with equivalent paces.
Predicted marathon
3:50:01
Breakdown
- Predicted 5K
- 23:59
- Predicted 10K
- 50:00
- Predicted half marathon
- 1:50:19
- 5K pace /km
- 4:48
- Half pace /km
- 5:14
- Marathon pace /km
- 5:27
- Easy training pace /km
- 6:45
Last updated:
FAQ
How does the Riegel formula work?
Peter Riegel's 1981 model scales a known finish time by the ratio of distances raised to the power 1.06. It captures the fact that doubling distance costs slightly more than double the time for most runners.
How accurate are race predictions?
Very good for races within a 2–3× distance band of your reference race, assuming similar training and terrain. Predicting a marathon from a 5K is a stretch — endurance training changes the equation.
Why is my easy pace slower than race pace?
The classic 80/20 approach keeps about 80% of running genuinely easy, around 60–90 seconds per km slower than threshold. The easy-pace figure here uses a common 1.35 multiplier as a starting point.
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