Riegel Race Prediction CalculatorFree online fitness & health calculator — instant results, no sign-up
Predict a race finish time from a recent race using the Riegel equation.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Riegel Race Prediction Calculator
The Riegel Race Prediction Calculator estimates a target race finish time from a recent completed race using the Riegel equation. It combines your completed distance and time with a target distance and an adjustable exponent that models how pace changes with race distance.
- Completed race distance (km)
- The distance of your recent completed race in kilometers, with a default of 10 km.
- Completed race time (minutes)
- Your total finish time for that completed race in minutes, with a default of 50 minutes.
- Target race distance (km)
- The race distance you want to predict a finish time for in kilometers, with a default of 21.0975 km.
- Riegel exponent
- A dimensionless factor controlling how predicted pace changes with distance, with a default of 1.06.
Formula and method
T₂ = T₁ × (D₂ / D₁)^k. With the default inputs: T₂ = 50 × (21.0975 / 10)^1.06 ≈ 110.32 minutes.
- T₂Predicted target race finish time in minutes.
- T₁Completed race finish time in minutes.
- D₂Target race distance in kilometers.
- D₁Completed race distance in kilometers.
- kDimensionless Riegel exponent; values above 1 model a slower average pace over longer distances.
Divide the target distance by the completed distance, raise this ratio to the Riegel exponent, and multiply by the completed race time. The default inputs give 50 × (21.0975 / 10)^1.06, producing a predicted finish time of 110.32 minutes when rounded to two decimal places.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Completed race distance10 km
- Completed race time50 minutes
- Target race distance21.0975 km
- Riegel exponent1.06
Results
- Predicted finish time110.32 minutes
- NoteApproximation; individual results vary.
How to use the Riegel Race Prediction Calculator
- 1Enter your completed race distance in kilometers; the default is 10.
- 2Enter the total time for that race in minutes; the default is 50.
- 3Enter your target race distance in kilometers; the default is 21.0975.
- 4Use the default Riegel exponent of 1.06, or adjust it if your race history supports a different value.
- 5Calculate and read the predicted finish time alongside the approximation note.
Reading your result
The “Predicted finish time” row estimates total elapsed race time, not pace: the default example gives 110.32 minutes for 21.0975 km based on a 50-minute 10 km race and an exponent of 1.06. The “Note” row states “Approximation; individual results vary.” This means the estimate is a planning aid rather than a guaranteed result or a measure of uncertainty.
Limitations
The equation assumes the completed race reflects your current fitness and that the target race involves comparable effort, conditions, and adequate distance-specific preparation. It does not account directly for hills, weather, altitude, fueling, pacing, injury, or changes in training. Predictions become less dependable when extrapolating between very different distances, particularly from short races to endurance events. The exponent of 1.06 is a common starting point, not a personalized constant. Both distances and the completed time must be positive.
Common questions
What does the default prediction tell me about a half marathon?+
A completed 10 km race in 50 minutes, a target distance of 21.0975 km, and an exponent of 1.06 produce a predicted finish time of 110.32 minutes. This assumes suitable half-marathon preparation and broadly comparable racing conditions.
How does changing the Riegel exponent affect the prediction?+
For a target longer than the completed race, increasing the exponent increases the predicted finish time. For a shorter target, increasing it decreases the predicted time. If both distances are equal, the predicted time equals the completed time regardless of the exponent.
Can I use a training run instead of a completed race?+
You can enter its distance and time, but an easy or interrupted training run usually understates race capability. A recent, representative race effort provides a more useful baseline.
Why does the calculator not simply scale my finish time by distance?+
Simple proportional scaling assumes you can maintain the same average pace at every distance. The Riegel exponent models the typical slowing of pace over longer races; an exponent of 1 would instead give proportional scaling.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.