Race Time Predictor: Estimate Your Finish Time
Enter a recent race time and distance to predict your finish time and pace at another distance, using the proven Riegel formula.
Predicted finish
- Pace / km
- 4:41
- Pace / mile
- 7:33
- From distance
- 5.00 km
- From time
- 22:30
| Split | Cumulative time |
|---|---|
| 1 km | 4:41 |
| 2 km | 9:23 |
| 3 km | 14:04 |
| 4 km | 18:46 |
| 5 km | 23:27 |
| 6 km | 28:09 |
| 7 km | 32:50 |
| 8 km | 37:32 |
| 9 km | 42:13 |
| 10 km | 46:55 |
How the Riegel formula works
Pete Riegel proposed the formula T2 = T1 × (D2 / D1)^1.06 in 1981. T is finish time and D is distance. The 1.06 exponent reflects the fact that pace slows slightly with distance, not in a flat line but in a predictable curve. For most well-trained runners the prediction lands within a few percent of the actual race result.
Riegel works best when the two distances are within a factor of about three: 5K predicts 10K and half marathon well, half predicts marathon reasonably well, mile predicts 5K well. Predicting a marathon from a mile time is a stretch and almost always over-optimistic.
How to use the prediction
Treat the predicted time as a target if your training is matched to the race distance. If you haven't been running long runs, knock 5-10% off a half marathon or marathon prediction. If you've been doing specific distance work, the prediction is a fair goal.
The pace output is just as useful as the finish time. Use the per-km or per-mile pace as your anchor for race-pace intervals and tempo runs. A 1:45 half marathon is 4:59/km or 8:01/mile, which gives you something specific to train.
Worked examples
Example 1. 5K in 22:30 predicts 10K in 46:53. Almost exact for a trained runner.
Example 2. 10K in 45:00 predicts a half marathon in 1:39:35. Achievable if you've done the long runs.
Example 3. Half marathon in 1:40:00 predicts a marathon in 3:28:39. Realistic with marathon-specific training, optimistic without it.
What Riegel doesn't account for
- Course profile. A hilly marathon is slower than a flat one.
- Weather. Heat, humidity and wind all matter.
- Fueling. Past about 90 minutes, nutrition becomes a limiter.
- Specific training. Marathon performance depends on long runs the formula has no way to see.
- Race-day pacing. Going out too fast can cost minutes on a marathon predicted to the second.
Common mistakes
- Treating it as a guarantee. It's a prediction, not a contract.
- Predicting marathon from a 5K. Too big a jump, the result is almost always overestimated.
- Ignoring fitness changes. A 5K time from six months ago doesn't reflect today's fitness.
- Skipping the long run. No formula can save you from undertraining the actual distance.
FAQ
What is the Riegel formula?+
Pete Riegel published it in 1981: T2 = T1 × (D2 / D1)^1.06. It assumes endurance scales predictably with distance, and it works well for races within roughly 3× of each other.
How accurate is the prediction?+
Within 1-3% for similar distances (5K → 10K). Less accurate jumping from short to very long (mile → marathon), where it tends to over-predict because true marathon performance depends heavily on specific training and fueling.
Why over-predict for big jumps?+
Riegel assumes you've trained for both distances. A fast 5K runner who hasn't done long runs will hit the wall in a marathon long before Riegel says they should.
Which exponent should I use?+
The default 1.06 fits well-trained runners. Some sources use 1.07-1.08 for less trained athletes. The calculator uses 1.06 by convention.
Can I use this for cycling or swimming?+
Riegel was built for running and is calibrated to running fatigue. It's a rough guide for other endurance sports but less reliable.
How do I use predicted pace?+
Run training sessions slightly slower than your goal-race pace for most workouts, with shorter intervals at or above goal pace. The predicted pace gives you a realistic anchor.
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