How to use the race time predictor
- Choose the distance of a recent race: 1500 m, 5K, 10K, half marathon or a custom distance.
- Enter your finish time. Use a real, all-out race or time trial from the last 6–8 weeks.
- Read the predictions for 5K, 10K, 15K, half marathon and marathon, calculated with two different methods, plus your VDOT score.
The two methods
Riegel formula. Published by engineer Peter Riegel in 1977: T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 captures how much you slow down as the distance grows. It is simple and works well between 5K and the half marathon.
Daniels VDOT. Coach Jack Daniels and physiologist Jimmy Gilbert built equations for the oxygen cost of running at a given speed and for how long you can sustain a share of your VO2 max. Your race gives a VDOT score, and the predictor finds the time at each distance that gives the same score. The same VDOT also sets your training paces in the training zones calculator.
When both columns are close, the prediction is robust. When they differ, the truth is usually somewhere between them.
How accurate are the predictions?
Predictions assume you are equally trained for both distances. That is rarely true:
- From a 5K to a half marathon, predictions are usually within 2–4% if you run 30+ km a week.
- From a 10K or half to a marathon, predictions are often too optimistic for runners who have not done long runs of 28–32 km.
- Heat, hills and a crowded start slow you down. Use a flat, cool race as your input.
Example predictions
| 10K time | Predicted half (Riegel) | Predicted marathon (Riegel) |
|---|---|---|
| 40:00 | 1:28:15 | 3:04:00 |
| 50:00 | 1:50:19 | 3:50:01 |
| 60:00 | 2:12:23 | 4:36:01 |
Next steps
- Check which Spanish half marathons fit your predicted time with the time limit checker.
- Turn your goal time into splits with the pace calculator.