The formula behind the prediction
Peter Riegel's endurance formula, published in the early 1980s, predicts a time at one distance from a known time at another: T2 = T1 × (D2 / D1)^1.06, where T1 is the known time over distance D1, and T2 is the predicted time over distance D2. The exponent 1.06 captures the fact that pace slows as distance increases — if a runner could hold the same pace at every distance the exponent would be exactly 1.
Because the relationship is a power law, the prediction compounds: the further D2 is from D1, the more the 1.06 exponent inflates the predicted time relative to a simple pace multiplication. This is why predicting a marathon from a 5K stretches the formula the hardest.
Predictions from a 25:00 5K
Applying the formula to a 25:00 (1,500-second) 5,000-metre time gives the following predictions across common race distances.
| Distance | Predicted time | Implied pace |
|---|---|---|
| 5K (known) | 25:00 | 8:03 / mile |
| 10K | 52:07 | 8:23 / mile |
| Half marathon | 1:55:00 | 8:46 / mile |
| Marathon | 3:59:47 | 9:09 / mile |
Why the marathon prediction is the least reliable
Riegel's formula assumes the runner is equally well trained for every distance. That assumption usually holds between nearby distances — a 5K time predicts a 10K well — but breaks down for the marathon, where success depends heavily on endurance factors the formula cannot see: weekly mileage, long-run history, fuelling, and the risk of hitting the wall in the final 10 km.
As a rule of thumb, a runner whose training is genuinely marathon-specific may match or beat the prediction, while a runner with fast short-distance speed but low mileage will usually run slower than predicted. The formula gives a ceiling that good endurance training earns, not a time that raw 5K speed guarantees.
Using predictions sensibly
Predictions are most useful for setting realistic pace targets and for comparing recent race performances, not as a promise. The closer your prediction race is to the goal distance — for example, using a recent half marathon rather than a 5K to predict a marathon — the more reliable the estimate. Predictors are best treated as one input alongside your actual training paces and long-run performance.
Часто задаваемые вопросы
What formula do marathon time predictors use?
Most use Riegel's formula: T2 = T1 × (D2 / D1)^1.06. It scales a known race time to another distance using an endurance exponent of 1.06.
What marathon time does a 25-minute 5K predict?
Riegel's formula predicts a 3:59:47 marathon from a 25:00 5K — but only if endurance training supports it. Low-mileage runners typically finish slower.
Why is my predicted marathon time faster than my real one?
The formula assumes equal training for all distances. Short-distance speed without high weekly mileage and long runs usually produces a slower actual marathon than predicted.
Which prediction is most accurate?
Predictions between nearby distances (5K to 10K) are most accurate. The closer your input race is to the goal distance, the more reliable the estimate.
Источники
- Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285-290.
- Riegel PS. Time predicting. Runner's World, 1977 — origin of the 1.06 endurance exponent.
- USA Track & Field — standard race distances (5K, 10K, half marathon, marathon 42.195 km). https://www.usatf.org/