CARDIO · RACE PREDICTION
Race Time Predictor
Predict finish times across 5K, 10K, half marathon, and marathon from any known race result using Riegel's formula.
Try a preset
Result
Predicted Finish Times
Riegel: T2 = T1 × (D2/D1)^1.06 — longer races get progressively harder per km.
How to use it
- Select your known race distance and enter your finish time precisely — even 30 seconds of error shifts all predictions. Use an official chip time, not a watch start/stop.
- Choose your target distances. The predictor uses Riegel's formula (T2 = T1 × (D2/D1)^1.06), which assumes consistent aerobic fitness across distances. It performs best when predicting to distances within 3× of your known race.
- Read the difficulty delta column: values above +5% indicate the longer race demands meaningfully more aerobic capacity per km than your baseline. A marathon typically shows +12–15% harder pace than a 5K.
- Use predicted pace for training zones, not just race goals. If the calculator says your marathon pace is 5:45/km, that pace is a useful aerobic threshold reference for long runs.
- Rerun after each race — your fitness evolves and a fresh data point is always more accurate than extrapolating from a 6-month-old result.
Questions people usually ask
What formula is used?
Riegel's endurance model (1977): T2 = T1 × (D2/D1)^1.06. The 1.06 exponent captures the non-linear fatigue increase with longer distances — you slow down proportionally more as distance increases.
How accurate are these predictions?
Within ±5% for well-trained runners predicting to distances within 3× their baseline. Accuracy drops for first-time marathoners extrapolating from a 5K, or for athletes who are significantly stronger at one distance.
Why does my predicted marathon time seem too slow?
Riegel's formula assumes consistent aerobic fitness across all distances. If you are undertrained for the marathon (short long runs, not enough weekly volume), your actual time will be slower than predicted. The formula models potential, not current race readiness.
Can I use a training run instead of a race?
You can, but add 5–8% to your training time to account for the difference between race effort and a typical training effort. Race results give the most accurate predictions because they represent maximum aerobic output.
Is the tool free and private?
Yes. All calculations are client-side. No data leaves your browser.
Related Resources
Learn the decision before you act
Every link here is tied directly to Race Time Predictor. Use the explanation, formula, examples, and benchmarks to pressure-test the calculator output from first principles.
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