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Race Time Predictor

The Race Time Predictor uses the Riegel formula to estimate how fast you could finish a race at a different distance, based on a recent race result. Whether you just ran a 5K and want to know your marathon potential, or you are targeting a half-marathon PR, this tool gives you data-driven predictions along with pace-per-mile and pace-per-kilometer breakdowns.

Quick Answer

The Riegel formula predicts race times as T2 = T1 x (D2/D1)^1.06. A 25-minute 5K runner can expect roughly a 52-minute 10K, a 1:56 half marathon, and a 4:03 marathon.

These results are estimates based on general formulas and are not a substitute for professional medical advice. Consult a healthcare provider before making health decisions.

Enter a known race distance and time above to predict your finish times.

How the Formula Works

  1. Enter your known race distance and finishing time.

    D1 = known distance (km), T1 = known time (seconds)

  2. Apply the Riegel formula to predict finish time at the target distance.

    T2 = T1 x (D2 / D1) ^ 1.06

  3. Calculate pace per kilometer and pace per mile for each predicted distance.

    Pace/km = T2 / D2 Pace/mile = T2 / (D2 / 1.60934)

  4. Review predictions across all standard race distances from 1 mile to 50K.

Methodology & Sources

Reviewed and updated April 4, 2026 · Prepared by GetHealthyCalculators Editorial Team

This calculator implements the Riegel formula (1977), one of the most widely used race time prediction models. It was derived from world-record data and validated across recreational runners. The formula works best when the known race was a maximal effort run within the past few months under comparable conditions.

References

  • Athletic Records and Human Endurance · American Scientist (Peter Riegel, 1981)
  • A Model for the Prediction of Performance in Running · Runner's World / Riegel (1977)
  • Predicting Running Performance: A Comparison of Models · Journal of Sports Sciences

Limitations

  • The Riegel formula assumes the known race was run at a maximal, even-paced effort — a training run or poorly-paced race will skew predictions.
  • Predictions become less reliable when extrapolating to much longer or shorter distances (e.g. predicting a marathon from a 1-mile time).
  • Factors such as terrain, weather, elevation, nutrition strategy, and race-day conditions are not accounted for.
  • The formula does not adjust for individual differences in endurance vs. speed — some runners slow down more than average at longer distances.

Frequently Asked Questions

What is the Riegel formula?
The Riegel formula is a mathematical model published by Peter Riegel in 1977 that predicts race performance at one distance based on performance at another. The formula is T2 = T1 x (D2/D1)^1.06, where T is time and D is distance.
How accurate are these predictions?
For well-trained runners extrapolating between similar distances (e.g. 5K to 10K), predictions are typically within 1-3% of actual performance. Accuracy decreases when predicting across very different distances or for runners whose training is skewed toward a specific distance.
Should I use my most recent race time?
Yes, use the most recent maximal-effort race time from the past 2-3 months. Older results or easy-effort runs will produce less accurate predictions. The input race should have been run under normal conditions on a flat course for best results.
Why is the 50K prediction included?
The 50K (31.07 miles) is a popular entry-level ultramarathon distance. While the Riegel formula is less accurate for ultras due to additional fatigue factors, it still provides a useful ballpark estimate for runners considering their first ultra.
Can I use a custom distance as my known race?
Yes. Select "Custom" from the distance dropdown and enter your race distance in kilometers. This is useful for non-standard race distances like 4-mile races, 15K events, or other distances not in the preset list.

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