Race Time Predictor
Predict 5K to marathon finish times using the Riegel formula
Race time predictor
Finish times across distances from one result, by the Riegel model.
How the race time predictor works
The predictor uses the Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06. It takes one performance you have actually run and scales it to other distances with an exponent that captures how pace falls as distance grows. Enter the distance and time of a recent race and the predicted times for 1 km, 5 km, 10 km, the half marathon and the marathon appear together.
The prediction is only as good as the reference. A 5 km run last week predicts a 10 km well; it predicts a marathon poorly, because nothing in five kilometres says how you cope with three hours. Use the race nearest in distance to the one you are planning.
The formula assumes the endurance to match — a runner whose lactate rises early fades more over the long distances than 1.06 allows. LaChart's race predictor inside the app corrects for that using your lactate curve and training history rather than a fixed exponent.
Questions
- Why does the marathon prediction seem too fast?
- Riegel's exponent was fitted on well-trained runners with the mileage for the distance. With a lower weekly volume the real fade is larger; many runners find an exponent of 1.08–1.10 nearer the truth for the marathon. Fuelling and heat can add more.
- Which reference race should I use?
- The most recent one, at the distance closest to your goal, run in reasonable conditions on a similar course. A hilly, hot 10 km predicts a flat, cool half marathon badly in both directions.
- How do I turn a predicted time into a pace plan?
- Divide the time by the distance for an even pace, then bank nothing: most good races are run at an even or slightly negative split. The heat and altitude calculator on this page adjusts the target for conditions.
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