By Zencalcy Editorial TeamLast reviewed About us
Predict how long a target race will take from a recent race result. The calculator uses the Riegel formula, which scales your time by distance with a fatigue exponent, and then shows the predicted pace you would need to hold.
Predict a race time from a recent result using the Riegel formula.
T2 = T1 × (D2 ÷ D1)^1.06 predicted pace = T2 ÷ D2
The exponent 1.06 encodes how fatigue builds as distance grows. The prediction assumes you are equally trained and raced both distances hard, so it works best when the recent race was a genuine effort.
| Symbol | Meaning |
|---|---|
| T1 | The time of your recent race, in seconds. |
| D1 | The distance of your recent race. |
| D2 | The distance of the race you want to predict. |
| T2 | The predicted finish time for the target distance. |
A 50:00 10K predicts a half marathon of about 1:50:19 by the Riegel formula. If you are not yet trained for the longer distance, treat the prediction as a target to build toward rather than a guarantee.
The calculator estimates a time at a new distance from a known race result using the Riegel formula, where the predicted time equals the known time multiplied by the ratio of the new distance to the old distance raised to the power 1.06. The exponent reflects the observation that running gets disproportionately slower as distance grows.
Doubling the distance, for example, does not double the time. Raising two to the power 1.06 gives about 2.09, so a run twice as long takes roughly nine percent longer than the simple doubling, which matches the way endurance fatigue accumulates over longer efforts.
The formula assumes the two races are run with similar training, terrain, and conditions, and that the runner is equally prepared for both. It works best for distances that are reasonably close, such as a 5k and a 10k, and it becomes less reliable when the gap is large, for instance predicting a marathon from a 5k.
It also ignores the specific demands of the longer distance. A runner strong in speed but light on endurance will tend to underperform the prediction at long races, while a high mileage runner may beat it. Use the number as a well reasoned starting point and adjust with real training results.
Continue with the Running Cadence Calculator, Running Pace Calculator, or the Running Split Calculator — or browse more running tools and all sports calculators.
A simple ratio would assume you can hold the same pace at any distance. The 1.06 exponent adds the expected slowdown from fatigue as the race gets longer.
Every result is computed locally in your browser from the formulas shown above. Nothing you type is uploaded, logged, or stored, and no account is required.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026