How it works
The predictor uses Pete Riegel’s formula: T₂ = T₁ × (D₂ / D₁)1.06. The exponent says how much a runner slows as the distance grows — a little over proportional, because fatigue compounds.
It is reliable from about 1500 m to the marathon for a trained runner. Predict from the race closest to the one you are aiming at: a marathon predicted from a 5K assumes endurance that only mileage builds.
A worked example
A 10K run in 48:30 predicts:
| Distance | Time | Pace per km |
|---|---|---|
| 5K | 23:16 | 4:39 |
| 10K | 48:30 (raced) | 4:51 |
| Half marathon | 1:47:00 | 5:04 |
| Marathon | 3:43:07 | 5:17 |
More free tools
- Race time predictor — a realistic time and pace for every distance from one race you have run.
- Running pace zones — six training paces from one recent race, anchored on threshold.
- Cycling power zones — six Coggan power zones from Functional Threshold Power.
- Swim pace zones — critical swim speed from a 400 m and a 200 m time trial, and four pool paces.
Pricing · Compare with Trainerize, TrueCoach, Everfit and TrainingPeaks · Free tools