Race Time Predictor
Riegel's formula converts a known race time into equivalent times at other distances. It works well between adjacent distances and over-predicts the marathon.
Your results
Predictions assume you are equally well trained for every distance, which is almost never true. Longer predictions are a ceiling rather than a forecast.
Riegel's formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06
The exponent 1.06 encodes the fact that pace slows as distance rises. It was fitted to race results across a wide range of distances and performances, and it is remarkably good between adjacent distances — a 5 km time predicts a 10 km time well.
Where it breaks down
- The marathon. Riegel assumes equal preparation, and the marathon punishes insufficient endurance work far more than shorter races. A good 5 km runner with 40 km a week will not hit the predicted marathon time.
- Very short distances. 1500 m depends heavily on anaerobic capacity, which does not scale from longer races.
- Untrained extremes. Predicting a marathon from a 1500 m time is stretching the formula well past what it was fitted for.
A practical rule: trust predictions within a factor of about three in distance, and treat anything beyond that as optimistic. For the marathon specifically, add 5–10% unless you have done the long runs.
Common questions
How accurate is a race predictor?
Good between adjacent distances. It over-predicts the marathon for anyone who has not done the endurance volume.
Why is my predicted marathon time too fast?
Because the formula assumes equal training for the distance. Marathon performance depends on long-run volume that shorter races do not test.
Can I predict a 5 km from a marathon?
Yes, and it tends to under-predict — marathon runners often have more short-distance speed than their marathon time suggests.
What is a better marathon predictor?
A recent half marathon time doubled, plus 6–10 minutes, is a well-known rule of thumb that accounts for the endurance requirement better.
Related calculators
Track the numbers, not just calculate them
MyFitsy stores your weight, measurements and every session, so the figures above become a trend instead of a one-off.
