Riegel Race Time Prediction Calculator
Peter Riegel's endurance formula predicts your finish time at a new race distance from a result you already have. It multiplies your known time by the ratio of the new distance to the old distance, raised to a fatigue exponent (the classic value is 1.06). The exponent above 1 reflects the universal fact that pace slows slightly as distance increases. This calculator returns the predicted time in seconds and minutes, the implied pace, and lets you tune the exponent to match your own endurance profile. Predictions are most reliable when the two distances are close.
Riegel formula
Predicted time = known time * (target distance / known distance) ^ exponent
Default exponent = 1.06
Pace at target = predicted time / target distance
Both distances must use the same unit so the ratio is dimensionless. The predicted time comes out in the same time unit as your known time. A higher exponent predicts more slowdown at longer distances.
Using the prediction wisely
- Accuracy is best when the known and target distances are close, for example 5K to 10K.
- Marathon predictions from short races tend to be optimistic; specific long-run training matters.
- The exponent is tunable; fit it to several of your own results for personal accuracy.
- The U.S. Centers for Disease Control and Prevention recommends gradual progression in endurance training to reduce injury risk.
- Treat the predicted time as a goal ceiling on a good day, not a guarantee.
Riegel prediction: frequently asked questions
What is the Riegel race prediction formula?
Peter Riegel's formula predicts a finish time at a new distance from a known time at another distance. It states that predicted time equals known time multiplied by the ratio of the two distances raised to a fatigue exponent, usually 1.06. The exponent above 1 captures the fact that pace slows as distance grows.
Why is the exponent 1.06?
Riegel found that an exponent of about 1.06 fit a wide range of endurance running results well. It means that doubling the distance increases time by a factor of two to the power 1.06, roughly 2.08, so you slow down slightly per mile as distance rises. You can adjust the exponent here if your own data suggests a different value.
How accurate is the Riegel prediction?
It is most accurate when the two distances are reasonably close and you are well trained for endurance at the longer distance. Predicting a marathon from a 5K, for example, tends to be optimistic because it ignores fueling, the wall, and long-run specific endurance. Treat the result as a target ceiling rather than a guarantee.
Can I use any distance units?
Yes. Because the formula uses the ratio of the two distances, the units cancel as long as both distances use the same unit. Enter both in miles, both in kilometers, or both in meters, and the predicted time comes out in the same time unit as your input time.
Does a lower exponent make predictions faster?
Yes. A lower exponent assumes you hold pace better as distance increases, producing a faster predicted time at the longer distance. Well-trained endurance athletes sometimes fit an exponent slightly below 1.06; less endurance-trained runners often fit a higher one.
Official sources
- USA Track & Field: running and coaching resources.
- U.S. Centers for Disease Control and Prevention: Physical Activity.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.