Race time predictor
Enter a recent race or time trial to estimate your finish time at other distances.
Predicted finish times
How it works
Predictions use Riegel’s formula. Finish time grows slightly faster than distance, by a power of 1.06.
When it runs optimistic
The formula holds up well to the half marathon. Predicting a marathon from a 5K assumes you’ve trained for the marathon. Without regular long runs, expect to be slower.
See a marathon planPredictions in the app
RunBuddy updates your race predictions from your PRs after every run.
Pace calculatorHow the prediction is calculated
The predictor uses Riegel’s formula: T2 = T1 × (D2 ÷ D1)^1.06. T1 is the time you entered and D1 is that race distance. D2 is the distance you want a prediction for, and T2 is the predicted time.
If time grew in step with distance, the exponent would be 1. At 1.06 your predicted time grows a little faster than the distance, because you slow down the longer you go. The predicted pace gets slower as the distance goes up. For a shorter distance the same formula gives a faster pace.
Worked example: half marathon from a 50:00 10K
The half marathon is 21.0975 km, so D2 ÷ D1 = 2.10975. Raised to the power of 1.06 that is 2.2064. Multiply by 3,000 seconds (50:00) and you get 6,619 seconds, which is 1:50:19. That is 5:14 /km. The same 10K predicts a 5K of 23:59 and a marathon of 3:50:01.
10K time to 5K, half marathon and marathon
Equivalent finish times from a recent 10K. The marathon column assumes you have trained for the distance.
| 10K time | 5K | Half marathon | Marathon |
|---|---|---|---|
| 40:00 | 19:11 | 1:28:15 | 3:04:00 |
| 42:30 | 20:23 | 1:33:46 | 3:15:31 |
| 45:00 | 21:35 | 1:39:17 | 3:27:01 |
| 47:30 | 22:47 | 1:44:48 | 3:38:31 |
| 50:00 | 23:59 | 1:50:19 | 3:50:01 |
| 52:30 | 25:11 | 1:55:50 | 4:01:31 |
| 55:00 | 26:23 | 2:01:21 | 4:13:01 |
| 57:30 | 27:35 | 2:06:52 | 4:24:31 |
| 1:00:00 | 28:47 | 2:12:23 | 4:36:01 |
| 1:02:30 | 29:59 | 2:17:54 | 4:47:31 |
| 1:05:00 | 31:11 | 2:23:25 | 4:59:01 |
| 1:07:30 | 32:23 | 2:28:56 | 5:10:31 |
| 1:10:00 | 33:34 | 2:34:27 | 5:22:01 |
Ready to train for it? See the 10K plan, half marathon plan or marathon plan.
Questions
Can I predict a half marathon from a 10K?
Yes. This is where the formula works best. A 50:00 10K predicts a half marathon of about 1:50, and a 45:00 10K predicts about 1:39.
How accurate is a race time predictor?
It is a good guide when you have trained for the target distance and the race you enter was a real, all-out effort. It is accurate to a few minutes at the half marathon, and less reliable the further you move from your input distance.
Can I predict a marathon from a 5K?
You can, but expect to be slower. A 5K tests speed, and the marathon also needs endurance. Without regular long runs, add several minutes to the predicted time. A recent 10K or half marathon gives a better marathon prediction.
What if my race was hilly or hot?
The formula only sees the finish time. A slow time from a hot or hilly course will predict times that are slower than you can run on a flat, cool day. Use a flat race or a time trial if you have one.