Predict your finish time for a target race distance from a recent race result, using the Riegel model, with predicted pace per km and per mile and a table across all standard distances.
Pick the distance of a race you have recently finished.
Pick the race you want a predicted finish time for.
Custom distances (kilometres)
| Distance | Predicted time | Pace / km | Pace / mile |
|---|
Every row uses the same known race as the input. The blue row is the race you entered; the green row is your chosen target. Predictions are most reliable when the two distances are close.
Each dot is one standard distance from the table above. The line rises because the Riegel exponent of 1.06 makes longer races slower per kilometre, not just longer in total.
For beginners: how to read this prediction
To get a prediction, start from a preset or enter your own race: choose the distance you have recently raced, type its finish time in hours, minutes and seconds, then pick the target distance you want a time for. The headline result is your predicted finish time for that target.
How the prediction is calculated
The calculator uses the Riegel endurance model. The predicted time equals the known time multiplied by the ratio of the two distances raised to the power 1.06: predicted time = known time × (target distance ÷ known distance) 1.06. The exponent 1.06 is a fatigue factor. A straight ratio would assume you hold the same pace at any distance, but real runners slow down a little per kilometre as a race gets longer, so the exponent above 1 stretches the predicted time accordingly. From the predicted time the calculator also derives pace per kilometre and pace per mile, using 1 mile equal to 1.60934 km.
Why this is not a pace calculator
A plain pace calculator divides a known time by a known distance to report pace for that same distance. This tool does something different: it predicts a finish time for a distance you have not raced, accounting for the way endurance pace changes with distance. Dividing a 10K time by ten and multiplying by 21.0975 would underestimate a half marathon, because it ignores the extra fatigue of the longer effort.
The cross-distance table
From a single input the calculator builds a table predicting finish times for every standard distance: 5K, 10K, 15K, half marathon (21.0975 km) and marathon (42.195 km), each with pace per kilometre and per mile. The row matching your entered race is highlighted as your known result, and the row matching your chosen target is highlighted separately, so you can see the whole spread at once. A small curve shows how predicted pace per kilometre rises with distance, which is the core idea of the model.
How accurate it is
The Riegel model is most reliable when the known and target distances are reasonably close and you are trained for both. Predicting a 10K from a 5K is dependable; predicting a marathon from a 5K stretches the model a long way and should be read as a rough ceiling. The calculator flags predictions where the two distances differ by more than a factor of three.
What is not included
This is a statistical estimate. It does not account for your training history and long-run preparation, course terrain and elevation, heat, wind, race-day fuelling, pacing strategy or individual health. Use the predicted time to set a realistic goal and pacing plan, then adjust on race day.