Race Predictor Calculator
Predict race finish time using the Riegel endurance formula.
This race predictor calculator estimates your finish time for a target race distance based on a recent race performance at a different distance, using the Riegel endurance formula - a widely used method for cross-distance running predictions.
Enter a recent race time and distance, plus your target race distance, and the calculator returns a predicted finish time.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 10K in 50:00, predict half marathon (21.1 km) | ≈1:50:20 | T2 = 50 × (21.1 ÷ 10)^1.06 = 50 × 2.11^1.06 ≈ 50 × 2.2065 ≈ 110.3 minutes ≈ 1:50:20. |
| 5K in 22:00, predict 10K | ≈45:52 | T2 = 22 × (10 ÷ 5)^1.06 = 22 × 2^1.06 ≈ 22 × 2.0847 ≈ 45.86 minutes ≈ 45:52. |
The Riegel Formula
T2 = T1 x (D2/D1)^1.06, where T1 and D1 are your known time and distance, and D2 is your target distance. The 1.06 exponent reflects that pace naturally slows somewhat over longer distances due to accumulating fatigue, rather than assuming a runner could hold their shorter-distance pace indefinitely.
Why Predictions Work Better for Similar Distances
The Riegel formula tends to be more accurate when predicting between reasonably similar distances (like a 10K predicting a half marathon) than for very different distances (like a 5K predicting a marathon), since factors like fueling strategy, pacing discipline, and endurance-specific training become increasingly important over much longer distances than the formula alone can capture.
How to Use the Race Predictor Calculator
- Open the Race Predictor Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the race predictor calculator works.
Frequently Asked Questions
How does a race time predictor calculator work?
It uses the Riegel formula, T2 = T1 x (D2/D1)^1.06, applying a known race time and distance to estimate a finish time at a different target distance.
Why does the formula use an exponent of 1.06 instead of scaling times linearly?
The exponent accounts for the natural slowdown in pace over longer distances due to accumulating fatigue - a purely linear scaling would incorrectly assume a runner could maintain their shorter-distance pace indefinitely.
Is a race time prediction accurate for very different distances?
Predictions tend to be more reliable between similar distances (like 10K to half marathon) than across very different ones (like 5K to marathon), since endurance-specific factors like fueling and pacing strategy become more important over much longer races.
How do I use this race predictor calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Race Predictor Calculator calculate?
Predict race finish time using the Riegel endurance formula. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this race predictor use?
The formula is: T2 = T1 * (D2 / D1)^1.06. Exercise physiologist Pete Riegel published this formula in a 1977 paper titled 'Athletic Records and Human Endurance,' after analyzing world record performances across a wide range of endurance distances and sports (not just running) and finding they consistently followed this same power-law relationship - the 1.06 exponent reflects the universal, gradual slowdown in sustainable pace as endurance events get longer.
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Last updated: September 27, 2026.