Race time predictor
Race time predictor
Enter one recent race result and see what it predicts for the 5K, 10K, half marathon and marathon.
| Distance | Predicted time | Pace |
|---|---|---|
| 5K | 23:59 | 4:48 /km |
| 10K | 50:00 | 5:00 /km |
| Half marathon | 1:50:19 | 5:14 /km |
| Marathon | 3:50:01 | 5:27 /km |
How the prediction works
The predictor uses Riegel's formula, which assumes you slow down by a predictable amount as races get longer: each doubling of distance takes a little more than double the time. The 1.06 exponent comes from fitting real race results across distances.
It works best when your input race is recent, run all-out, on a course and in conditions similar to your target race.
When predictions break down
- Marathon from a 5K. Speed doesn't guarantee endurance. Without long runs, the prediction will be too fast.
- Hills and heat. A hilly or hot race can cost minutes the formula can't see.
- Durability. If your heart rate drifts a lot late in long runs, you'll fade in the back half. Relic measures this as aerobic decoupling.
Relic's running analysis tracks a VDOT from your real race efforts over time, so you can see whether your predictions are moving.
Questions
Riegel's formula is usually within a few percent between 5K and the half marathon when your input is a recent all-out race. Marathon predictions are optimistic for most runners unless they've done marathon-specific long runs, so treat them as a best case.
Peter Riegel's endurance model: T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your race time over distance D1 and T2 is the predicted time over D2.
No. Use a race or a time trial run at full effort. A comfortable training run will under-predict what you can do on race day.