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Race Time Predictor
Predict what you could run at a new distance from a recent race result. Enter the distance and time you ran and the distance you are targeting; the calculator applies Riegel’s formula and lists equivalent times from the mile to the marathon.
- Pace per mile
- 8:23
- Pace per km
- 5:13
Equivalent times at standard distances
| Distance | Predicted time | Pace per mile | Pace per km |
|---|
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Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Choose the race you ran recently and enter your finish time as h:mm:ss or mm:ss.
- Choose the distance you want a prediction for.
- Read the predicted time and pace, and check the table for every standard distance.
Worked examples
A 25:00 5K projected to 10K
- Predicted time
- 52:07
- Pace per mile
- 8:23
- Pace per km
- 5:13
A 50:00 10K projected to a half marathon
- Predicted time
- 1:50:19
- Pace per mile
- 8:25
- Pace per km
- 5:14
A 25:00 5K projected to a marathon
- Predicted time
- 3:59:47
- Pace per mile
- 9:09
About Riegel’s formula
Engineer and runner Peter Riegel published the formula in 1977 after observing that pace slows by a steady proportion as race distance grows. The exponent 1.06 means that doubling the distance multiplies your time by about 2.08 rather than 2.
It works best between distances that are not too far apart — 5K to 10K, or 10K to half marathon — and when you have trained for the target distance. Predictions from a 5K to a marathon are usually optimistic for recreational runners, who slow more over 26.2 miles than the formula assumes unless their weekly mileage is high. Use a recent, all-out result on a similar course.
Questions people ask
What 10K time does a 25-minute 5K predict?
52:07, a pace of 8:23 per mile. Riegel’s formula multiplies the 5K time by 2^1.06, about 2.085.
What marathon time does a 25-minute 5K predict?
3:59:47 by the formula. Treat that as a best case that assumes thorough marathon training; many runners finish well behind their 5K-based prediction.
Why is the exponent 1.06?
It is the value Riegel fitted to performances across distances. An exponent of 1.00 would mean no slowing at all with distance; larger values mean more fade. Some coaches use 1.07 or 1.08 for less-trained runners.